Just for another "data point:"
I posted about this here last year, wondering what would work best for those of us who commute by bike and want to combine that with race training. What I've found is that I really have no energy in the evenings, so I almost always do my hard workouts, sprints, intervals, in the morning, then spin home. Just like most of the others here seem to do.
But I also make absolutely certain that I take at least two days, even three, off the bike each week. Because riding twice in a day means your body only has half the time to recover for next day's ride as it would if you rode only once a day. (BTW, I ride between 120-200 miles/week, with 20 miles to work each way plus a longer ride or a race on weekends.)
Men's Health magazine's current issue (June '06, Hugh Jackman on the cover) has some brief thoughts on twice-a-day workouts, FYI. Seems to have a place in good workouts in their opinion.