What do you mean with compatible? Work in both XHTML 1.1 browsers and XHTML5 browsers? Be conforming to both at the same time?
The answer to the former is yes since XHTML5 should be compatible by design.
The answer to the latter is probably also yes; XHTML5 is more relaxed in some parts and more restrictive in others. XHTML 1.1 requires a doctype declaration and it is optional in XHTML5. XHTML 1.1 has a
bug with the usemap="" attribute that is fixed in XHTML5, so if you want to be an
asshole you can't use client side image maps and conform to XHTML 1.1. There are probably some other differences but they would be pretty minor.