All advice aside, I'd rent a tri bike for a day and see if you like it. If so,commute to work on anything that is comfortable to you. The contrarian in me says it might be cool to tool up to work in something other than a road bike.

I have a road bike (Trek Madone), a cyclocross bike (S-Works CruX), and a 10 pound wonder (Emonda SLR 10). I ride the Madone almost every day because of its ridiculous comfort, and then the CruX on rain days, and then the Emonda when I just want to haul ass and race to work. I have race light tires on the Madone and the Emonda.