You need to upgrade to 5.1.1 of Studio to get WAS 5.1.x support. You can up the UTE in WAS 5.0 to any of the 5.0.x fixpacks. The major difference with 5.1 of WAS is WAS is that it is based on JDK 1.4.x so it is a totally different J2SE. Upping to Studio 5.1.x gives you support to both WAS 5 servers and WAS 5.1 servers.
Roland Barcia: IBM Distinguished Engineer, CTO Mobile for Lab Services