Computational feasible to reverse MD5SUM? This might be out of ignorance, I apologize, but how complex of a problem might it be to generate a file of size $N$ whose MD5SUM is $X$?

For example, 

ls -l wczasp.rb
-rwxrwxrwx  1 master master     7273877 2011-01-30 17:13 wczasp.rb

md5sum wczasp.rb
acf3602b5eb9a2db3e365d3043682faf  wczasp.rb


So, how computationally hard might be to generate any 7273877-byte file whose 128-bit checksum is also acf3602b5eb9a2db3e365d3043682faf? In a sense, I am asking for a hash collision. My intuition says next to trivial, and nearly as useless too, but I would like to know for sure.
Actually, to the best of our knowledge, it's computationally infeasible.

By the terminology what we use when we discuss cryptographical hash functions, you're not asking for a hash collision (which is "find two different messages that hash to the same value"), but instead you're asking for a hash preimage (which is "for this hash value, find a message that hashes to it").

While MD5 is known to be woefully inadequate when it comes to resistance to hash collisions, there is no known way to find hash preimages that is faster than brute force (that is, hash different message until you stumble across one that happens to hash to the target value).  This brute force approach with MD5 will take $O(2^{128})$ expected operations, and so it is computationally impractical.