Should fall be capitalised in the following? If yes, is it because Fall 2011 is a proper noun?
Where should an app be released in Fall 2011?
In a Wikipedia article, Avatar (2009 film), a particular summer is capitalised (my emphasis):
Work on the language for the film’s extraterrestrial beings began in Summer 2005, and Cameron began developing the screenplay and fictional universe in early 2006.
(The related question Should the names of seasons be capitalized? is about the seasons themselves (as in I love the colors of the leaves in autumn.), not a particular season, like Fall 2011.)
I’m not really interested in OP’s should. He can choose which style guide he wants to follow, and take their advice.
Obviously if the season itself is part of a compound name (an event, academic term, publication issue, as per @Gnawme’s examples), it’s capitalised just like the other words.
As regards what people actually write in other contexts, instances of last summer,summer 1995 in Google Books suggest people usually capitalise the latter, but not the former. I believe this is because even though both terms reference a single specific season, including the actual year makes it more of a “proper noun” with a single referent which never changes.
By contrast, last summer is a more ephemeral concept – last year it was this summer, and by next year it’ll need far more words to identify exactly which one we’re talking about. In such usages, the name of the season behaves more like a pronoun – we capitalise John, but not the more generic he (unless “He” is [a] God).