Bob Lutz has been quoted as saying they looked at 6 speeds in the solstice and found them to be no quicker 0-60 or in the quarter mile than the 5 speed.
The main reason to want a 6 speed, (with the base engine, anyway) would be for track racing, where you could use the closer ratios.
I also read in an article in the news section, that Pontiac didn't have the time or want to spend the money to use non off-the-shelf gears, and they realize that the drop from 2nd to 3rd is a little too big.
I'd bet that the 07's have this issue addressed, and maybe even an optional 6 speed.