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I believe there are some minor issues. I think changing all HTTP_POST_VARS, etc to the modern equivalents is the only big thing to do, which will be part of the 2.2 release.
One of our developers was running it in a local PHP 5 environment, but I think he had made the above change manually.
Compatible or not, Formulize is not written to take advantage of the new features in PHP 5.
--Julian