I've already mentioned that TaxCut Online works flawlessly under Linux 
(using Firefox) with only a one-time nag screen... but I found another 
online tax solution that *may* explicitly support Linux -- Tango.  It's 
another H&R Block product, but uses Flash this time.

I mention this only because of this blurb on their front page:

'windows, apples, penguins - we love u'

Okay, so one might be concerned about a tax product that doesn't spell 
out "you", but that's the first I've seen *any* of the majors actually 
make (in)direct reference to Linux.

http://www.hrblock.com/tango/

<disclaimer>I've never used that software so I don't know how well it 
works.</disclaimer>