I use a super low-tech, unsophisticated, non-nuanced formula to calculate calories burned for dieting purposes. It as follows:
100 cal/mile running
40 cal/mile cycling

Doesn't take into account speed, wind, hills, weight... but for basic dieting purposes (and quickly calculating whether a given snack is justified on a long ride), it has served me pretty well. If I were trying to plan nutrition for a serious event, I might want something a little more accurate...

Out of curiosity, let's see how that stacks up too:
For a 2 hour ride:
Chris Carmichael's calculation -- between 1,560 and 2,040
My calculation, assuming average of 20mph while riding (that is, not the actual average speed on your computer, but the speed you see most often) -- 1,600
Oakleaf's Garmin -- 1,700

There's some calculator online somewhere that lets you plug in your weight, the bike's weight, the wind speed, elevation gain, power output, whether you've shaved (just kidding)... and it makes an estimate for you... this might not be much less accurate than a GPS/hrm, given how many variables and fudge factors it seems there are...