I'm not sure this is specific to my iPhone because my Macbook also has same issue. But it's driving me nuts on my iPhone right now. Here is the issue... Sometimes my iPhone works fine on my wifi network and then out of the blue it will just swich to 3G and if I try to connect it back to the wifi it will say Unable to join the network "ssid". And sometimes it asks me to reenter the pre shared key, sometimes not. Usually I can get it to reconnect by turning the wifi off on the iPhone for a while, like maybe 20-30 minutes, then turn it on and much of the time it will then work. I suspect it's an issue in the Darwin code, common on both the Macbook and iPhone, because I've used this Linksys WRT54GL running DD-WRT 2.4 for years and this issue has only popped up in the last 6-8 months with my Macbook and now I'm really noticing it on my newly purchased iPhone 4. Noticed issue running both iOS 4.1 and 4.2. Issue does not seem to show up on my 3 Windows laptops. Only apple devices so far. And I have not noticed this issue when I use my iPhone 4 on my employer's Cisco wifi network. Why would these devices work fine for hours or days and then suddenly be unable to connect / reconnect??