the number of shares you'll find in 24 hours is:
86,400 / (difficulty * 2^32 / hashrate)
EDIT: Somethings not right there:
Using 1,400,000 for difficulty, and 3,200,000,000 hashes (3200 Mhash) I get 0.045980726 shares???
in Excel:
=86400/(1400000*(2^32)/3200000000)
THANKS! I feel I need to add
"If you're going for that degree of accuracy, it should be noted that difficulty 1 does in fact not correspond to 2^32 hashes, but to 2^48/65535 (4295032833 instead of 4294967296). – Pieter Wuille Sep 17 '11 at 12:21"