Perhaps all is not lost even after a format.
Google has a number of responses including this one which mentions formats.
http://www.gorecovery.com/digital_ph...ry_for_mac.asp Sandisk have a recovery program which they include with some of their flash cards. (Mine came with an 8GB card, unfortunately it is a small CD which I cannot use in my Mac) I would try their program, it may help.
PS, just noticed that this URL is for the Mac version but there is a link on the page for a PC version.
PPS. I am assuming that I should not have to mention that if you have overwritten the card with more images some of the previous data will likely not be recoverable, but then I am sure you knew that :-)
Yet another PS/edit but here is the link to the Sandisk program and it says it covers formatted cards.
http://www.sandisk.com/Products/Prod...o.aspx?ID=2195