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"