Happened to me right after the iOS5 update - lost all my Gmail calendar entries. Was disgusted, so re-created my calendar (and moved email and contacts, too) into iCloud's. Has been very stable ever since.
However, every once in a while, the month-view iCloud calendar will have some days blank. But that's usually after I've been very active with my iPad or iPhone. If I do a hard reset, all better (hold down power and home button until white apple shows up). On my PC, I use Safari to access my iCloud email, etc., and if the calendar has problems there, I close Safari and reopen it. Normally, that takes care of it.