We changed the way VM Options are loaded when starting the Mashup Server in 1.5 and recently found out that these are not loaded properly in the Windows core startup script (wso2mashup.bat). This might lead to Java Heap Space errors once you run the server for more than 24 hours, with a few heavy duty Mashups running.
The best fix is to get the script from our SVN repository (revision 16554) and replacing your local copy. Upon restarting the server, all should be fine. You can test whether the script works (if you have further doubts) by changing the values found in the mashup.vmoptions file.