Canon has just about standardized the SD series of cameras.

The only noteworthy difference is that the SD850 has a 4X optical zoom and the SD1100 has a 3X optical zoom.

They both use the same CCD sensor, engine and flash system.

The images they produce should be identical.

Here is a specification comparison:

Note: I spotted two errors in the SD1100 chart:
The Effective Pixels should read 8.0 million (not 7.0).
The Image Stabilization should read Yes, Lens (not Yes, CCD).

You can click on the green links just under the photos to see an In-Depth Review.