E! Entertainment Channel belives that Whose Line is not canceled.
In a report published May 22nd, 2003 the EOnline website reports that Whose Line will be used as a mid summer replacement and that The Drew Carey Show will also be on the bench to come in when needed. Check out that report here.

Whose Line not on the Fall 2003 ABC Line Up
By some accounts (TVGUIDE.COM) Whose Line is over, canceled. By other accounts (abcmedianet.com) it is simply not on the schedule yet for next fall. And by yet another account, (the futon critic) ABC has yet to make up it's mind. Here we go again. Here's the letter....again. I changed it up a little from the last time.