Most applications currently running on Oracle Solaris 10 will continue to run unmodified on Oracle Solaris 11 Express, but you wanna be sure. Some applications could have incompatibilities.
Oracle Solaris 11 Express Compatibility Checker allows developers to zero in on incompatibilities and fix 'em. The Checker includes:
Checkers are available for SPARC and x86 versions of Solaris. Get the README here.
If you have questions or comments about the tool, send them to the friendly folks at firstname.lastname@example.org.