The N900 does not work with WPA and DHCP connections. If you have the option to use WPA2 on your access point, then it's better to use that instead - it works correctly.
Hey im just informing you guys, that im having the same problem, but i can connect my n900 to my home wifi if my computer isn't on / disconnected to modem, but when my pc is on / connected to modem, my n900 cant retrieve ip address when i try to connect wifi. what could the problem be?
Is your PC and/or your N900 configured with a static IP address? Are you using DHCP in the router? if yes, have you "fixed" the IP address for any devices? What is the range of IP addresses that your DHCP provides?