I too have this problem. I have an iPhone 3G runing 3.0.
I set up my pc to constantly ping the iPhone ip address. Once the iPhone connects I get about 6 ping responses but after that all I get is time outs. If I leave the Wi-Fi Neworks menu up on the iPhone I notice that after I connect a few seconds later the "Chose a Network..." shows a loading symbol and after that point I get no ping responses. Does anyone know what it is doing after it connects? I can only speculate it might be going in a power saving mode or searching for new networks, what ever it is I suspsect that is the cause of the problem.
But I have never been able to connect to my router - I have gone through the trouble shooting guide at http://support.apple.com/kb/TS1398 and googled the problem. I assume the problem is with the iPhone since it is the only thing that does something prior to disconnecting from the network (as described above). My router is an MSI RG54GS - with latest firmware, is there a list of known routers that work with the iPhone? This would be handy as atleast then I would know if I am chasing my tail.
Another question I have is there a a list of "prefered" radio settings for the iPhone? I am talking about settings on the router e.g. Beacon Period, DTIM etc.
Any help muchly appreciated, I have spent several days trying to connect my iPhone to my home Wifi and am very dissapointed I cannot get it working.