Yes, and a very tough one to make.

Actually, it's not that hard.

The wiki states somewhere (can't find the page now, of course) that the probability of a single hash calculation solving a block is

      target
     -------
       2²⁵⁶

and the average number of hashes required to solve a block is its inverse:

       2²⁵⁶
     -------
     target

Let's call that last formula A. On average, then, it takes A hashes to solve a block. We know how long it takes to create a particular block (just take its creation timestamp and subtract the timestamp of the previous block from it). So the network hash rate is simply A timespan.

Of course, individual blocks will be highly variable due to the random nature of finding a solution (there were about 3 or 4 blocks recently that were created within 5 minutes; the block before that series took 20 minutes, as did the block after). To get a more accurate estimate, you'd want to take the time over 50 or 100 blocks. The formula is:

      2²⁵⁶ * n
-----------------------
target * (ts_N - ts_N-n)

where
n is the number of blocks you're sampling
ts_x is the timestamp of block number x
N is the latest block number (or whatever block number you want to end your sample range with)
and target is currently 0xCBBD000000000000000000000000000000000000000000000000