While I'm not so sure i think the heart there was intentional I did find this on a facts page on amazon.com:
There were 75 visual effects shots created for the film by the Canadian house Buzz Image Group. Of these, 15 were of CGI sheep. The film called for about 2,500 sheep, but only 700 were on-set, necessitating the additional woolly creations. Also created for the film were sky replacements, set additions, erasures and the hail in the hailstorm.
It does say sky replacements, this I'm sure could be for a number of reason, most probably being due to weather or something as subtle as a moon phase (now that i think of it I'll have to pay attention to if there are any moon shots, maybe in relation to phases of their relationship...) but off hand I can only think of one scene early on with Jack...