All the DHC-8/100's are gone. The last one was pulled from service just last week. Six F-28's will be around til 2003 as N400QX has mentioned.
The DHC-8/200's will be around for many more years. I think going by the 10 year lease factor the first woukld end in 2007...extensions?? Who knows.
Personally, I wouldn't be surprised if the Q400's went away before their smaller counterparts. Nor would I be dissappointed....memories of the Dornier 328 come to mind.