Over the weekend I installed Windows7 and connected to my Apple Time Capsule (802.11n only 5GHz) network during the install, everything was working fine
Then used Microsoft Update and downloaded the 'optional' Dell 1510 wifi drivers (Broadcom provided 22.214.171.124 @07/07/2009) which stopped my connection to the 802.11n network working. Testing showed I could connect to my slower 802.11g network running off a Buffalo dd-wrt router, so the driver was working just not at 802.11n speed.
Playing around with the Properties|advance tab I switch 'AP Compatibility Mode' from 'Higher Performance' to 'Broader Compatibility' allowed to get back on the faster network; speed is reported as 300Mbps.
This might help solve your issue.