I think it's a problem with 10.5. My MBP did that occasionally after connecting to our home network after using the MBP on other networks. Since upgrading to 10.6, I've not seen it recur.
There is one thing that helped me before upgrading the OS. Are you using the wireless feature? If so, do
System Preferences > Network and select the Airport card from the left pane. Then click the "Advanced...." button. The first tab in the resulting window should be "Airport." Look at the "Preferred Networks" pane.
It's likely cluttered with a lot of old networks from travels and form otherwise using the computer in cafes, etc. Delete any networks you do not use at least weekly. For example: leave your home network, the one at work, and the one for your favorite coffee shop and get rid of the others. To keep new acquisitions from building up, uncheck "Remember networks this computer has joined," or simply remember to clean out networks occasionally.
This does not affect your ability to join new networks later.
