Personally, I think the risk of an unencrypted C: drive is very small IF you have moved (a) the swap file; (b) the TEMP folder; and (c) your e-mail store (if it's local) off of C: to an encrypted drive. All three of those are simple to do. Most applications keep their data in your designated documents folder ... and any you use that don't are most likely configurable -- so you can easily ensure they also keep their data in an encrypted drive.
I agree, however, that's it's much simpler to not have to worry about that -- and simply encrypt everything (C: included). While that may create issues with Acronis, note that Windows 7's built-in Backup utility will easily create a system image ... and it supports BitLocker [not to mentioned that it's free :-) ]. Aside from "playing" with it to confirm it works (it does ... and quite well) I don't use it, as I have a complex multi-boot system with 8 OS's that I image fairly regularly -- I use an Image Set with Boot-It NG to do those images (but none of these are encrypted).