Gah! Almost thought I'd found something...
uwhois.com will search all the top-level domains, but (for example) searching for google.com results in a selection of "google.com.and.stuff.here.from.stupid.scammers.com" results.
Possibly still okay for me, but it's not really a reliable check.
:/
If I want to do a specific/exact search, I can only check 10 TLDs per request, and three requests in too short a period gets you locked out for a period.
:( Searching all 250+ would take /ages/
OOOH! I think
www.markmonitor.com is doing good. Still not an actual web service, but it's returning useful results.
EDITED: 15 Mar 2008 13:41 by BOUGHTONP