Just saw this in the Citrix MPS3.0 admin guide :)
ICA encryption (SecureICA). Use ICA encryption (Citrix SecureICA) to encrypt the information sent between a server running MetaFrame Presentation Server and a client. ICA encryption makes it virtually impossible for unauthorized users to open an encrypted transmission and, in the unlikely event that such an attack succeeds, ICA encryption ensures that the attacker sees only meaningless screen commands and not sensitive information. ICA encryption provides confidentiality to guard against the threat of eavesdropping.
Note ICA encryption is not recommended for use across public networks.
So secure ICA is awesome, but where you really want to use it, we don’t recommend it…