I've never do this before, but for Windows OS vmware converter seems to work fine, you must treat de source vm like a physical server during the conversion step. For Linux OS take a look at this ( http://www.google.com/url?sa=t&rct=j&q=convert+xen+to+vmware&source=web&cd=4&ved=0CL4CEBYwAw&url=http%3A%2F%2Fcommunities.vmware.com%2Fservlet%2FJiveServlet%2Fdownload%2F38-62151%2FXEN_VMware_migration.pdf&ei=DfjFT_nmDc3csgaH1ui0Aw&usg=AFQjCNFmtTgjzrQgPw1eGz5iQZsFUtSB8w )
If anyone have other experience about that, please post the result here 