From @whyrusleeping on Tue Aug 25 2015 19:04:59 GMT+0000 (UTC)
ipfs uses a sha256 hash for addressing content. Meaning that there are 2^256 different possible hashes. Lets assume that the entire bitcoin mining economy decides to try and find an ipfs object hash collision, checking hashes at a rate of 400 Petahash (400,000,000,000,000,000 hashes per second) it would take them 2.810^59 seconds, or 910^51 years to compute the entire space. Even factoring in the birthday paradox and moores law, we shouldnt need to worry about that happening for a few thousand years or so. Assuming sha256 is a perfect hash function (which it might not be).
If sometime down the road, we get worried about our very limited hash space, we can upgrade the hash function we use as ipfs uses the multihash format for specifying hashes along with their type and length.
tldr, the world will likely end before we find a sha256 collision. but if we get worried, we can use a bigger hash at pretty much any time.