[/quote]
.....

So if your miner does ~17THs then you'd expect to expend (3.9x10^21) / (17*10^12) seconds, on average, to find a block at the current difficulty
Which is about 233million seconds or about 7.4 years
But since difficulty changes, there no point in saying that silly large number, it's better to simply say that at the current difficulty, your 17THs has a 1 in 2699.7 chance of finding a block in 1 day, which makes sense and is correct to actually say that.

Of course it's random and you could find it today, or you could never find it.
Them's the breaks

I also have a web page that does these sort of calculations for you:
http://tradebtc.net/bitcalc.php
[/quote]

Wow!  Great explanation that did go a little over my head, but I get the gist of it and I can see the numbers getting huge very quickly.  I will settle in and wait for my dice to roll double 6's   

Thanks for the calculator link too, good stuff. 

Cheers