My Droid X2 does the same thing if it has been asleep for more than a few minutes. It wakes up immediately if you put it to sleep and then wake it up within a minute or so. I'm guessing that it goes into a power savings mode after a few minutes of sleep and requires a longer start up time from that state.
I left my bluetooth turned on for 24 hours and still have 75% battery left since the last charge, 24 hours ago. Only had 2 calls in that time for a total of 5 minutes air time. It must be doing some kind of power savings in sleep state.