I know this is cheating, but the new Citroen DS3 convertible is a 5 seater. Well, its officially a 5 seater, but less so a convertible..... and of course, the 3 in the back will need to be either very thin or very close ! :-)
The jeep wrangler came with 5 seat belts too, i seem to recall that the old PT cruiser convertible had 5 seats too, but then you would have to find 4 more people willing to be seen in one..
None of the standard crop are 5 seaters though !
Great Question !