Hi Mike,
As Deb explained a while back, it isn't possible to have LTM use the client's cert to establish an SSL handshake with the pool members.
http://devcentral.f5.com/Default.aspx?tabid=53&view=topic&postid=22280&ptarget=22289
There is no mechanism by which to directly forward the client's certificate via the standard authentication process, since using the client's cert to establish the session would require the LTM to use the client's private key as well. (A man-in-the-middle attack, basically)
You can instead use the session table to store the certificate & send it to the server via headers, assuming your app can pick it up from there. Here's an example from the iRules codeshare:
http://devcentral.f5.com/wiki/default.aspx/iRules/InsertCertInServerHeaders.html
HTH
/deb
Aaron