I had exactly this issue after installing Lion.
In the end I deleted my home network (network preferences) then rebooted and reconnected as if for the first time (including manual entry of passwords etc)
For me this solved it.
One other thought. Have you updated Lion to 10.7.2? There was a fix for this type of failure in 10.7.1.
If you have Lion, you should try this fix:
A clean install of Lion also would help as on one of my maching the wireless has not droped yet since XMas.