16GB is the largest I have seen.
However, it's unlikely that the camera can format that card for more than 8GB.
The Digic-II cameras seem to limit their formatting to 8GB. There are firmware updates for the "mark II" 1-series cameras to allow them to format more memory than this, but no firmware updates have been made available for the 20D/30D/5D and the Rebel series (350D/400D).
It's possible that the XTi/400D already has the updated firmware, but I doubt it. The updates for the 1-series came out just before Christmas, and the XTi/400D is still shipping with the same firmware that it had back in September.
Even without the updated firmware, the 1-series cameras can apparently use larger cards if they're preformatted, so I would guess that the same is probably true of the other Digic-II DSLRs. Also, as I understand it you could still format a 16GB card, it just comes out with only 8GB.
As for DSLRs with the original Digic chip (the DReb/300D in this forum), I don't know. My guess is that it's also stuck at 8GB.
Personally, I don't use anything larger than 2GB at the moment. The 1GB and 2GB cards are at the "sweet spot" for pricing, and having multiple cards means more flexibility and security. Also, the display of estimated shots remaining on the DReb/300D and XT/350D can't go above 999, and some people get really annoyed that they don't know what the estimate is.
