Aha! OK, so it turns out that my issue is NOT with ReportViewer, but rather with RSClientPrint. Once I did a google search for that I quickly found that the answer is: Upgrade to Sql Server 2008 R2 SERVICE PACK 2.
The version of RsClientPrint you get with R2/SP2 is 10.50.4000, while the version I had was 10.50.1600.
In conclusion, it appears that RSClientPrint 10.50.1600 is NOT COMPATIBLE with IE10, but version 10.50.4000 IS.
I REALLY hope this helps someone else!!