In a post here Jamie Thompson suggests extending the container object in SSIS to allow for code re-use and crucially allow more than one developer to work on one package at a time. This would be a big help on large projects. He's put in a Connect request which currently has 9 votes. If you want to see this feature in the next version of SQL vote now!
Personally I'd like to see better branching and merging support so that its easier to compare packages. There is Smart Diff, part of BIDS Helper which is better but it's:
a) an add-on
b) still needs a Source Control client
c) still forces you to do manual XML merges
One can but hope...