I had this problem with my Powerbook....never re-acquiring the network after awaking from sleep.
In order for your Mac to always look for the right network:
You need to enter the Network panel in System Preferences and then click on Airport, and Configure.
On the 'Airport' tab, change "By default, join..." from 'Automatic' to 'A specific network' and enter the Name/Password.
Problem should be solved.