I always have to reconnect. Even if it falls asleep, when it wakes up, it doesn't auto-connect to my home network. I have to do it manually. None of my other macs do this... most recent version of OS X LIon...
Mine has done this since day one. And I think quite a few people had this problem also.
I don't have to enter the password or anything, I just have to go onto my WiFi and re-select the network again. Sometimes I'll have to do it twice as the first time I get a "Connection timeout" error.
Mine remembers the network when plugged into power but if the MacBook has been sleeping unplugged it is almost guaranteed to have forgotten the network. Mildly annoying.