There seems to be a trend in Hollywood of releasing similar movies at similar times. I have never really understood this as one nearly always fails, and this may have not been the case it we didn't have something to judge it against.
The list of the near identical films includes:
Deep Impact vs Armageddon - both released 1998
Antz vs A Bug's Life - both released 1998
Dante's Peak vs Volcano - both released 1997
The Illusionist vs The Prestige - both released 2006
Capote vs Infamous - release 2005 & 2006 **but still too close for comfort**
and now we can add another to the list, as yesterday I saw two previews that up until then I believed to be the same film. The films in question are Friends with Benefits and No Strings Attached as you can guess by the titles, the films are about two best friends who decide to sleep together; no feelings required. No Strings Attached stars Natalie Portman and Ashton Kutcher and Friends With Benefits stars Mila Kunis and Justin Timberlake, both are booked in for a 2011 release. Neither seem great, although Ivan Reitman directs No Strings Attached and I really like Kindergarten Cop so i think i'll see that one.