How to secure OWA

A client with Exchange 5.5 (he is not thinking to upgrade), want to use OWA, but is concern about the security problems if it; so, needs some solution to secure the connection with the OWA. First we thought about Whale Communications but was just bought by Microsoft and the new products are not sold in Latinoamerica.

Which other solution is there, to secure OWA and also give access to it by WAP and for PDAs?