It seems to be working for me. I exported at 6 x 4 inches at 300dpi. Checked in CS3 and it 6 x 3.987 inches at 300dpi.
Looking at the Apple support boards it's one of those 'broken for some not for others' issues.
I've tried deleting and recreating the export presets but I still can't export JPEGS at 300 dpi.
I've this minute tried again:
The first screenshot is my 6x4 export preset: