You need to hash about 2^64 values to get a single collision among them, on average, if you don't try to deliberately create collisions. Hash collisions are very similar to the Birthday problem.
If you look at two arbitrary values, the collision probability is only 2-128.
The problem with md5 is that it's relatively easy to craft two different texts that hash to the same value. But this requires a deliberate attack, and doesn't happen accidentally. And even with a deliberate attack it's currently not feasible to get a plain text matching a given hash.
In short md5 is safe for non security purposes, but broken in many security applications.
