An article from The Register.
http://www.reghardware.co.uk/2009/06/09/iphone_3gs_o2/
Apple’s iPhone 3G S small print also contains some finer points you should be aware of because, despite version 3.0 of the OS set to launch on 17 June, many of the coolest features required hardware only included in the 3G S.
iPhone_3GS_02
For example, even if you upgrade your existing 8GB 3G to the new operating system, you still won’t get the compass, the voice control app, the video capture capable camera or the “improved performance” afforded by the 3G S' faster chippery.
Internet tethering will not be available to owners of the 8GB iPhone.