I had a similar problem with both of my (2008 Macbook) USB Sockets.
I ended up fixing the problem by shutting down my macbook and very gently bending the tiny metal usb grips (inside the usb socket) either with a tooth pic or a needle, as they had seemed to have become flat (due to unpluging devices regularly).
See this pic for info. http://tinypic.com/view.php?pic=xfvink&s=8#.VMoT9Fo_FyE
Good Working USB sockets should have the 2 metal prongs (circled in red) sticking up slightly, if they are flat then they won't grip the usb plug when inserted and will loose connection regularly (the prongs complete the circuit by possible providing a ground connection)
I also cleaned the connectors with a cotton bud/q-tip and some isopropynol alcohol.
Give it a try. Both my sockets are working fine now, no disconnections even when i wiggle the usb plugs inserted into the sockets :)
I hope this helps