First, what OS are you running? Leopard or Tiger?
Are you entering the IP address number as this "afp://192.168.1.12" or similar?
Are you sure file sharing is enabled in the Systems Preferences panel?
I do this all the time and it works perfectly. But it didn't once when I got the number wrong, so it may sound stupid, but recheck to make sure the address is correct. (Check the laptop'S Network Pref pane)
Can you connect using the finder?
These simple things should just work so there may be a simple answer, a bad ethernet cable etc.