I know what you mean. Prob & Stat are always confusing to me, and I always have to slog through some reading before I feel comfortable with writing anything about it.
I found how to compute the standard deviation, but either I'm doing it wrong (most likely), or it's pretty useless because of loss of precision (For the expected value, we subtract one huge number from another and magically get the result. It seems the same occurs with the standard deviation but the numbers are *much* larger, so for interesting problem sizes, the interesting bits get pushed off the right end of the float).
I currently don't have anything useful for standard deviation yet...
Update: I like the idea of knowing the standard deviation, though, as it may make it possible to estimate of the number of "positive matches" from your document & bloom filter project. ("Hmmm, we expect 200 false positives with a variance of 20, but we're seeing 400, so we've probably got about 180 to 220-ish matches.")