In a previous blog posting I mentioned the problems here and here, I was having with my 802.11n WPA2 encrypted network and a MacBook Pro.
In the end what seemed to help was ensuring that Airport was at the top of the network configurations.
However even with that it still took the MacBook Pro some time after sleep to re-connect to my 802.11n Airport wireless network. Even then sometimes it failed to re-connect so I had to turn the Airport on the MacBook Pro off and back on again.
The MacBook Pro has now been upgraded to Leopard 10.5.2 and this does seem to have resolved the issue fully, with the MacBook Pro re-connecting to the wireless network immediately rather than after a minute or so.
Hopefully it will remain resolved and not happen again.