I have had this annoying problem with my Macbook Pro. Sometimes when waking the machine from sleep, the machine can not connect to the selected Airport network and then hangs with spinning pizza of death.

Apples next update to 10.5.3 looks like it will fix it. Hopfully Apple will release it soon…

From Fixed list of Mac OS X update 10.5.3
– Joining AirPort network after wake issue resolved