We often experience problems using file sharing on MacBooks (running 10.5). The issue seems to be a bug in the firewall. We tend to keep file sharing disabled unless we are actually using it. When sharing is enabled, the proper hole in the firewall between local computers is not opened. To get around the problem, turn your firewall off, start your connection, then turn your firewall back on. This doesn't appear to me a problem on the PowerPC based Macs we have used.