Well, the most 'professional' way to take a screen capture is to buy a capture card or run a video signal into a PC and use video capturing software. (Frapps?) I have used a 'Dazzle Platinum' myself. (which forced me to play in standard definition and made me give up on my old youtube channel) That being said, if your looking for a screenshot for bragging rights, (or to make a how-to) I have often seen a digital camera used to take a picture of the screen. While this is not the ideal solution, if you decrease back light (glare) and turn off your flash its often legible.
These are the only two ways to do it at the moment. Some games are beginning to support in game 'montage video editing' (black-ops has this option) but until this technology matures your really only left with the two options. Capture the image through the wire, or capture the image through the display (tv or monitor).
I hope this helps