In Stargate: Atlantis, Season 1, Episode 8 "Underground," Major Shepard's team meet the Genii. This marks the first appearance of Colm Meaney (Chief O'Brien on Star Trek: TNG) as Cowen, leader of the Genii.
Near the end of the episode, Cowen orders his men from hiding to surround Major Shepard and his team. As one of those soldiers moves into place, we get a pretty tight close-up. I swear it's Patrick Stewart. However, I can't find anything from IMDB, Stargate's wiki, or other internet sources to confirm this.
Was that Patrick Stewart, or just a guy who looks like Patrick Stewart? This badly ripped video from youtube is all I could find to provide a link for, but pausing on my DVD gives a clear picture, and I swear it's him.