Well, unless you count Charles Van Doren and the Today show, I think Ken is probably the first contestant (in recent memory) to get a national commercial deal. Though, I have seen some of the former Survivor contestants get commercial deals. Usually nothing big with the national companies, but some local organizations. I remember Rupert being in a few Survivor promos, and Kathy (from Marquesas) did a commercial for an eye business in the Burlington VT area.
Probably the reason Ken Jennings got the big deal is not only because he spent an uber-long time on a quiz show, but also because the quiz show is extremely popular. You probably didn't see Thom McKee getting commercial deals because Tic Tac Dough was not a national phenomeonon like Jeopardy is today. Granted the show only received an average of probably about 10 or 11 in the ratings, but if you ask anyone in the country about the show Jeopardy, they'll know about it.