It's possibly the timeout setting (the power saving mode). I'm aware of only one free hotspot in my town which has the timeout problem, and when the timeout setting was set lower (as in the newer versions of OS2007) I don't see that router at the initial scans, it does show up after a while sometimes (but I can't connect).

However, if you upgraded from the latest version of OS2007 to OS2008 (or from an earlier OS2008) then you shouldn't see any difference. If you did then this could possibly be something else entirely.