Another thread reminded me  of this little diddy:

We think we are first introduced to Moriarty as Jim from IT in the lab with the infamous exchange between Sherlock and 'Jim'. However, I believe we see Moriarty very briefly just before this, as John and Serlock exit the cab and are about to enter 221B. 

The camera holds long, to allow the shot of the door to completely close and allow the extras to finish walking by. 

One of those extras looks like a deadringer for Moriarty with the distinctive sideburns arch, if not Moriarty himself. This shot is just before Sherlock finds Carl Powers shoes in 221C. The images are thumbnails, click for larger pics.

I'll leave you to your deductions.