I have an Arrive w/ Mango. It used to connect to my WiFi at home with no issues, but for the past week or so I've noticed it no longer connects and relies on the cellular network for data. It has no problem connecting to any other WiFi network.
There have been no changes to my access point and all other WiFi devices can connect with no issues. I've tried rebooting the phone, pulling the battery, deleting and re-adding my network in the list of known networks. On my router side I've rebooted it, checked and double checked the password, cleared all expired ARP tables, etc.
I don't know about your router, but most will allow you to back up the settings to your PC. Do that first, then reset your router to factory defaults and see if you can connect. If you can, then there HAS been some change to the router. You would have to start putting in your settings one by one to find the culprit. If you can't connect - have a friend bring their phone over. If they can connect then it must be the Arrive.
Well, I have three other phones in the house (Hero, Evo, Blackberry Tour) as well as two Touchpads, laptops, etc. and all connect with no problem. DHCP pool has plenty of IPs left to administer. I'll try a few other things and then give the router reset thing a try. It's a Netgear 3700 BTW.