When the iPhone 3GS came out late last year, commentators seemed to think it wasn't much different, so I didn't take any notice. Now that Handruin has objected to me belittling his pride and joy, I realize that in fact the 3GS (or possibly OS 3.0?) is a very big step up.
The Nokia 5800 was released in late 2008 and is now end-of-life. Consequently, it's very cheap and quite a bargain. As requested, here's the advantages that I know of:
1. Autofocus camera. I don't care about the 3 vs 2MP, but autofocus means it's actually useful closer than 6m/20ft away. Also has a decent macro function, for taking pics of bugs etc. * fixed on the 3GS
2. Video camera. Pretty basic feature. * fixed on the 3GS
3. Camera flash.
4. Second (facing) camera for video calls.
5. Can actually do video calls. * fixed on the 3GS
6. Voice recording. * fixed on the 3GS - I think?
7 Voice dialing. * fixed on the 3GS
8. Stereo speakers. Not as good as the amazing old Motorola E1000, which was like a mini boombox, but still way better than most phones.
9. AD2P stereo Bluetooth. * fixed on the 3GS
10. Bluetooth file sharing.
11. FM Radio. Stupid omission by Apple.
12. Micro-SD slot, comes with 8GB and accepts up to 32GB (16GB officially). * Now that the 3GS comes with 16GB as minimum, I think this has become less important. Indeed, Nokia's thinner successor has 16 or 32G hard-wired
13. 50% higher screen resolution (although the screen is slightly smaller).
14. Narrower body (51.7 vs 62.1mm) makes it much more suitable for women.
15. Lighter weight (109 vs 133g).
16. Removable battery. * At least in the 3GS, the battery is no longer soldered in! Still bad on a SmartPhone in particular
17. Miscellaneous functions such as Cut and paste, Saving email attachments, Message forwarding, Multiple SMS deletion, Sending SMS to multiple recipients and Multimedia messages (MMS 1.3). * at least some of these are fixed on the 3GS
18. Multi-tasking. Bit of a no-brainer, this one. Our only current phone that lacks this is a $25 Samsung.
19. It's not an Apple product. If you need it fixed, you can take it to a Nokia service center. Google returns 156,000 matches for "Nokia service sucks", 3,110,000 for "iPhone service sucks" and 4,950,000 for "Apple service sucks".
I mentioned the drawbacks earlier in this thread.
Don't get me wrong, I don't have one myself - although it's certainly tempting at the current pricing - but it does highlight how little you get for your money when you buy Apple.