It's now at 8 million shares and 16 hours @ 600ghash/s.
Mathematically very, very improbable, if not practically impossible
http://portal.acm.org/citation.cfm?doid=355993.355997
http://www.itl.nist.gov/div898/handbook/pmc/section3/pmc331.htm
(The probability of not solving a block within 960 minutes at current difficulty with 600ghash/s is <0.0000000000000001% and decreases exponentially after each passing minute)
I get a different answer. The chance of solving a block depends on difficulty.
Here is the probability of an individual hash solving a block: http://blockexplorer.com/q/probability
The current probability of solving a block with an individual hash is: 0.0000000000000001489590023459044440534704278888966655358
The current probability of not solving a block with one hash is therefore :
( 1 - 0.00000000000000014895900234590444 ) = 0.99999999999999985104099765409556
Let's call that chance of not solving with an individual hash (a number very close to 1, or 100%) "Z".
The probability of not solving a block with two hashes is Z * Z, better written as Z^2. Still nearly 100%. We need more hashes.
The pool averaged about 600Ghash per second. That is 600,000,000,000 hashes per second x 60 seconds per minute x 60 minutes per hour x 16 hours in the "imaginary" round. Probably around 34,560,000,000,000,000 hashes. That's another preposterous number, so I'll call that "hashTotal"
A pool trys hashTotal hashes in 16 hours, so the chance of not finding a block in that time is Z ^ hashTotal.
.99999... ^ 3.456E+16 = .02156 = 2.16% chance of not solving a block in 16 hours. Not improbable. Bruteforcing the nonce statistics don't enter into it, since everybody is doing random work on multiple blocks. Actual calculation done in Excel, not Matlab, so feel free to verify the percentage for me.