Microsoft SQL Server 2008 encryption over multiple DB versions
Requirement 2 requires the data be stored "normally": that is, you can't use encryption in the client for all data. Encrypting data in the client also contradicts requirement 1

Requirement 3 requires the media or the actual backups are encrypted. Encrypting the HDDs isn't reliable because once someone has the actual media then it can normally be unencrypted by a sysadmin.

So, I'd suggest using a 3rd party tool like Red Gate SQL Backup Pro or LiteSpeed by Quest to secure your backups. Points 1 and 2 are satisfied because the on-line database is unchanged