For those of us (foolhardily?) brave enough to upgrade to Tiger via Archive & Install, one of the annoying consequences can be the inability to select anything other than "A specific network" in the Network settings panel to join by default. The new "Preferred Networks" setting is also AWOL. (Another annoying consequence of an Archive & Install: Volume Logic doesn't move to the new installation ... drat!)
While this hint discusses general tips for cleaning out the Airport known/preferred network list, the solution for this problem is much simpler: delete and recreate the network locations that use Airport (Location menu -> Edit Locations... -> Delete). Voila! The Automatic & Preferred Network choices now show up.
Mac OS X Hints