Yup I also converted my Criterion DVD to a MKV container just yesterday - http://originaltrilogy.com/forum/topic.cfm/Converting-non-anamorphic-166-1-DVD-to-16-9/topic/15240/
I personally like the grain on this, so didn't bother with the cleanup script mentioned in the first post.
My TV is only 40 inch but this still looks beautiful to me.
Would love a Criterion Blu Ray version. I'd be more interested in getting a lossless soundtrack than HD video.