A list of potential compatibility problems is provided by Sun : http://java.sun.com/j2se/1.5.0/compatibility.html
Have you encountered other problems when running/testing old-1.4-applications against Java 5 (eg. A specific library no longer working ....) ?
In general, code that runs under 1.4 should work under 1.5. I don't think any previously deprecated methods were even removed. However, bugs are fixed between releases and you might get some slightly different behavior if your application was written in such a way that depended on the bug (I have encountered this with a Swing application).