were there major changes in WP 2.6.1 would make a template that works in 2.6 not work any longer? you can see here http://netentropy.com (it is 2.6.1 , widgets not working and in the backend the theme previewer is blank) but http://www.netentropy.com/blog (it is 2.6, everything works fine)
I doub't it, WP are very keep on backwards compatibility. However, I guess they will make a lot of stuff obsolete in WP3 as PHP4 has recently been discontinued. Also, WP doesn't use a template engine. It used PHP files, which is a bad and extremely insecure solution.