Use of RSS DTDs Deprecated
Chris Finke: until July 1, 2007, the DTDs for RSS 0.9 and 0.91 will be available via my.netscape.com
Given this, it makes sense for the Feed Validator to issue a warning.
Update: In the past 24 hours, 7 additional click-throughs to this message were performed; this level of activity would not qualify as a Common Feed Error. Interestingly, two chose to correct this — by removing the DOCTYPE.