Since I upgraded my Airport Extreme network to a pure WPA2 802.11n 5GHz wireless network, my MacBook Pro has consistently failed to re-connect to the network after waking from sleep.
It does this in both 10.4.10 and 11.5.1 and when connected to an Airport Express running 802.11g WPA/WPA2 no problems, will re-connect every time.
My iMac which is connected to the 802.11n network does not have this issue.
If I move back to 802.11n/b/g on the 2.4GHz radio mode the problem disppears.
I have followed the advice given by Apple here and another piece of advice which said ensure the Airport is at the top of the network configurations, but alas no luck.
Once or twice the MacBook Pro has reconnected, but 99% of the time no re-connection and the only solution is to either join the 802.11g network or re-boot.