I've now had a few minutes to play with the new iPhone, which is due out in the UK on June 19th (the same day as the Nokia N97 and two days after the iPhone 3.0 software update).

On the outside, the iPhone 3G S is a dead ringer for the iPhone 3G. I find it surprising that Apple didn't make any changes to the physical appearance - after all, it would have encouraged a lot more iPhone 3G owners to upgrade. Perhaps they're holding something back for Christmas. Or maybe Apple just didn't want to raise expectations - the 3G S is, as its name suggests, a tweak of the existing handset rather than a new phone.

So, what's changed? There are 4 key differences: speed, camera, voice control and compass.

Speed
With a beefed up processor and hardware acceleration, the 'S' is noticeably speedier than the 3G. This is a blessed relief because, while the original iPhone was pretty nippy, with each software update it has become marginally less responsive. But on the S, switching between applications is instant, and I didn't experience any painful lags.

Amazingly, this extra power doesn't come at the expense of battery life - the 3G S now offers 9 hours of Wi-Fi web browsing (compared to 6 hours on the old handset); 10 hours of video (vs 7 hours); 30 hours of audio (vs 24 hours); 12 hours of 2G talk (vs 10 hours). 3G talk time is unchanged at 5 hours.

Camera
The new 3MP autofocus camera still lags behind the competition in terms of resolution, but the tapping on the viewfinder to set focus and exposure is a nice feature (though not original - see HTC Touch HD). There's still no flash, either, but the iPhone has always made a pretty decent job of shooting in low light conditions and my brief test in the darkened backstage room at WWDC confirmed that the 3G S makes even more better of limited light.

Nonetheless, many of the iPhone's critics won't be placated by its stills capability.



The video camera works nicely, capturing VGA video (in H.264 with AAC audio) and automatically adjusting frame-rate (between 15fps and 30fps) based on light conditions. Once you've captured some video you can do some very basic editing by setting in- and out-points and hitting 'Trim' (this is a destructive editing process, so you will lose the clips you cut).

The camera software won't allow you to cut in the middle of a videoclip, or join clips together, but it's likely that third-party apps will allow more sophisticated editing.

Once you're happy with your edit, you can upload to YouTube or MobileMe with a single button click, or send by email or MMS. Most networks have an MMS limit of 600kb, so the iPhone will compress the video before sending it. Alternatively, you can copy the video across to iTunes or iPhoto when you sync your phone. It's smart enough to  know if you've recorded video in portrait format, which is the natural way of recording video on the iPhone - but it'll be interesting to see how YouTube will deal with portrait-mode videos.

Voice control
I'm not convinced that voice control - and the addition of shake-to-shuffle motion control - is anything more than a gimmick. It is, however, a rather cool gimmick. Hold down the home button or the call button on your headset, and you can ask your iPhone 3G S to call any of your contacts, tell you what's playing on the iPod or even set up a new playlist based on your current song.




Voice control works in over 30 languages, but it'll be interesting to see how accurate it is when you have lots of names in your contact book, and 32GB rammed with music. It doesn't learn your voice, so will it really be able to tell 'AC/DC' from 'Hayseed Dixie' or 'Ryan Adams' from 'Bryan Adams'?



Compass
The addition of a compass is hardly revolutionary, but it means that by double tapping on the 'location' button in Google Maps, your iPhone 3G S will tell you which way you're facing by rotating the map and showing a beam that represents your field of vision.

Interestingly, a compass isn't necessary for the turn-by-turn navigation, which calculates your direction based on movement. Which is good news for current iPhone 3G users who want to buy the new TomTom navigation app.



iPhone 3.0 software
You don't have to buy a 3G S to use the iPhone 3.0 software - it's a free upgrade for iPhone owners and £10 for iPod Touch users. But it does add some really cool features to the iPhone - most significant are:
- Tethered web access that lets you use your iPhone's 3G connection on your laptop over USB or Bluetooth. Alas, it'll cost £15 extra a month from O2
- Find my iPhone. Use any web browser to show where your phone is on a map and activate an alert sound so you can find it - or, if you can't, you can remotely wipe all of your personal data.
-  Peer-to-peer gaming and content sharing over Bluetooth or the internet.
-  Turn-by-turn navigation.
-  And, I suppose, MMS. Picture messaging never got my juices stewing, but at least it's an option now.

Conclusion
The 3G S addresses the iPhone's biggest flaws without providing a great leap forward. It'll be hard justify upgrading from an iPhone 3G unless you're desperate for the extra capacity (which will certainly be my excuse).

But together with the iPhone 3.0 software, the 3G S provides an even stronger platform for the app developers who have already turned the iPhone from a mere mobile into a social phenomenom. And while some decent rivals are finally appearing, the likes of the Palm Pre, Nokia N97 and HTC Magic are going to find it difficult - if not impossible - to match the iPhone's momentum.

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Apple iPhone 3G S -  O2 pricing details revealed
Live reports from the launch of the iPhone 3G S