Folks,
I've had the same problem and found a perfect solution which is in play right now as I speak... just buy 3 RCA "splitter/combiners"... found out about on the hometheaterspot.com under forums. I purchased from RadioShack.com (don't have them in store). They were only 3.95 each. Both the Xbox and DVD player are attached to the combiner and then into the TV. Works GREAT, with no visible signal loss. I just turn off the Xbox when I want to Watch DVD and vice versa...
Saved me a bunch of money from having to buy a "switcher box"...
good luck
Joe