GNOME 2.x has had a dictionary since the good old 1.x days. The current Dictionary is a DICT protocol client, relying on existing dictionary servers like dict.org. This is sub-optimal, as the DICT protocol really only works for ANSI 80x25 terminals (the text is pre-formatted); doesn't have markup; doesn't allow translations; and it's rapidly fading out of the fact of the Interwebs.
The GNOME Dictionary should remove the DICT client support and switch to being a local search front-end to Wikipedia/Wiktionary.
This would also be the chance to redesign the Dictionary UI to be more in line with the rest of the GNOME 3 applications.
Design in progress
Implementation in progress
Dictionary (GNOME 3.1)
Fantasdic (GTK 2.x application)
StarDict (GTK 2.x application)
OS X Dictionary