There is a good convesation on the BPOS Meet up Group about Exchange Online and encryption. In short, you can get encryption services for as little as $9.40 a month that works with Exchange Online. That allows 5 simultanous users. That means that if you had 200 users, any 5 of them could be using the services at the same time. http://www.microsoft.com/online/exchange-hosted-services/encryption.mspx
Check out this post:
• Any BPOS customer may purchase a minimum of 5 Encryption services at $9.40 per month. Once BPOS sets it up at MS, anyone in that domain, up to 5 simultaneously, may use the Encryption service. In other words, if there are 100 users in the BPOS domain, any of the 100 users may use encryption because it is based on active (encryption) users, in this case 5. If you or anyone would like to see it in action I can put together a Live Meeting and demo.