This is a bit of a mess, but I got mine up and running. There is some type of conflict going on with the Evo and secured WEP networks. Here's what I did that got mine back up and running:
I tried all the rebooting, updating, and forget network trips. None worked. I did not want to do a hard reset on my Evo. Sprint sent several "flash updates" to my phone, but none worked.
I then focused on the router and cable modem. I reset them by disconnecting for 5 mins. Both router and modem save/keep settings so I knew I wouldn't have to do a lot to get them back up in running. Then, I set my Evo to forget my network. When I turned the router and modem back on, I went into the router settings, changed it to default, then changed the password back to the one I wanted to use (My wife's Evo had no issues while I had my wifi woes, and she didn't even realize I'd done anything because I essentially set it back to the same connection we had before my issues appeared.).
Sprint was not able to help. I spoke to 3 Sprint reps. The last one said, "Sorry, I'll put in a ticket and maybe someone at a repair store can fix it." I asked if they thought this was server related, and they all said "it must be the phone." It wasn't the phone - at least not exactly.
I'm not in IT, but I know enough to know that there was a communication problem between the phone and router. I just couldn't tell which was the real problem. Seems that 1) Evo does not "forget" like it should and that there has to be some cached information that doesn't allow the router to see it as a new device (mine was a new Evo); 2) Resetting the router without resetting the cable modem didn't work - I tried doing just the router and it did not work any of the 5 times I tried; and 3) Resetting the password on the router seemed to help because the router would not take my password until AFTER I had it cleared and re-entered.
I hope this helps someone avoid the full day of frustration I've just gone through.
I never had a chance to go check an open wifi. I wouldn't have done that anyway despite recommendation from Sprint. I told them the only wifi I actually care about is my secured, home wifi. I also told them that not being able to access wifi is a failed feature of the phone and that I expect a better remedy than "we don't know." 2 of 3 reps were very courteous and tried to be helpful, but I think they just don't have the ability to go outside basic help.
Good luck.