Quite a few reasons (besides something wonky in your setup):

Some major internet providers (as well as some universities) block p2p--see http://www.slyck.com/news.php?story=590

The lists of peers accepting new connections (including the gwebcaches) are getting overloaded. see http://groups.yahoo.com/group/the_gdf/message/20836

The new beta version (4.1.7) is trying a new method of getting connected. See if that helps.
http://www.limewire.com/english/content/beta.shtml

Otherwise, see if a friend who is connecting successfully will send you a recent copy of their "gnutella.net" file to replace yours. See
http://www.gnutellaforums.com/showth...955#post101955