There are past links [1][2] for multiple domain SSO. But if you will to take a look at recent release of v11.1 SSO configuration guide, it now support inherently (w/o iRules) multi-domain support and also section talking about common use cases for SSO in APM (e.g. for LTM, thru network access with layered VS, access web appl directly) [0]. May be useful to take a look at it.
[0] BIG-IP® Access Policy Manager® Single Sign-On Configuration Guide
http://support.f5.com/kb/en-us/products/big-ip_apm/manuals/product/apm-sso-config-11-1-0.html
[1]Single APM session across multiple FQDNs
http://devcentral.f5.com/wiki/APM.ShareAccessCookies.ashx
[2]Multi-domain SSO using NTLM
http://devcentral.f5.com/wiki/APM.MultipleNTLMSSO.ashx