A first draft of the 2011 MLB schedule -- not anywhere near finalized -- was shared with ESPNBoston.com recently.
Buried in that link is this nugget:
The [Red] Sox are scheduled to play teams from all three National League divisions in interleague play. The Chicago Cubs are scheduled to visit May 20-22, Milwaukee is in June 17-19, followed by San Diego June 20-22.
Presuming this schedule holds -- and I don't see any reason it wouldn't, with this many details having leaked already -- that will be the Cubs' first regular season visit to Fenway Park and first since they played Red Sox in the 1918 World Series.