ACS doesn't support SAML (the protocol). It is confusing, because WS-Federation uses SAML Tokens too.
On a side note: if your app is only going to authenticate users in your CA SiteMinder, then there's not much value in in having an intermediary (ACS). If you can turn on WS-Fed in SM, then just connect the app to it. WIF only supports WS-Fed, by the way, so you'll need WS-Fed regardless (or use some other library besides WIF).
An intermediary is valuable when you have multiple identity sources: your SM, someone else's, etc. and can act as a broker of authentication.
Depending on the protocols you need to support ACS might or might not be a good candidate (do you need OAuth? SAML? WS-Fed? etc).