DD_545i

+1 ... second that ! It's one of the best features of an automatic. It's why they're so great in traffic jams, parallel parking, creeping slowly to the end of the garage without destroying the bikes on the back wall but still going far enough that the garage door will close (ok maybe this one's a bit too specific to me ).

If mine stopped creeping, I'd be visiting BMW and asking for my creep back.

OverSoft

Like noted above, it is a standard "feature" of a automatic box with a torque converter.
What actually happens (warning, physics content):

Your engine is idling and is driving the torqueconverter (TC).
The torqueconverter is (put very simply) a liquid clutch.
There are two discs in the TC, one is connected to the engine output shaft, one is connected to the automatic tranny.
The TC is filled with oil (basicly, standard transmission oil) which starts spinning because of the idling output shaft of the engine.

When the transmission is in neutral or in drive and you let go off the brake, the other disc in the TC starts spinning because there is less friction to keep it in place.
You start creeping forward, because the engine is still producing some power when it's idling, so that in turn spins the TC, which drives the transmission, which drives the wheels.

The TC is also the reason you can accelerate while your RPM is stable. (Push the gas, RPM is at 3000, you start accelerating with little to no RPM change)
It takes time for the other side (the tranny side) to get up to the speed that the engine is doing.

(To read more: Wikipedia)

Nothing to worry about! (Reasons it might NOT be creeping is a high amount of friction on the wheels, the tranny or the TC itself, i would get this checked)