Here's the most simple way to explain it.
If you use any program that accesses the GPS, after that time, the phone will no longer go to "sleep", even if you fully close (or even force close) the app that started the GPS.
Typically. when you hit the power button or let the screen timeout, the phone will go into a power saving sleep mode shortly thereafter.
In the case of the Droid X, after a GPS program is used (assuming that user doesn't then toggle off the GPS entirely), the phone never goes into sleep mode and uses more battery power than it should.
Open Spare Parts, click on Battery History, third line down is "running". You'll see that gets pegged at 100% after the GPS has been accessed. That means the phone is never sleeping. You should plug in the phone and then unplug just before your test so the counters reset.
Someone at Motorola needs to investigate this ASAP. It seems to be a major bug that's decreasing battery life for all Droid X users