I suspect that the most accurate bike-ready accessory would be a power meter.

My bike computer has an altimeter and HR, and it reports higher KCal burnt than something like Endomondo (Strava doesn't make a BlackBerry 10 app...)

Take the actual value with a grain of salt, and if you really need to measure against something, use it as a relative metric - if it says you've burnt more Calories today than yesterday's within the same app, you probably did use more energy.

I wouldn't rely most of these things for actual Calorie counting.