We reviewed the possibility of supporting Tomcat 5.5 in Roo generated applications but came to the conclusion that it is better to recommend for Roo users to upgrade to Tomcat 6. Tomcat 6 (stable) was released more than 3 years ago and all feature improvements are included in the 6.0 releases. As you noted Tomcat 6 does also support the JSP 2.1 API whereas Tomcat 5.5 does not.
Roo makes extensive use of the JSP 2.1 API in the scaffolded Web UI (specifically expression language). Furthermore, the JSP 2.0 API does not support JDK 5 enums (a feature that Roo would need).
Therefore, the current scaffolded views do not run on Tomcat 5.5. It may be possible for you to develop a custom tagx library which could be used with the scaffolded Roo views (a feature that will be included in the upcoming Roo 1.1 M1 release) which are JSP 2.0 API compliant. This would however be done by an external addon.
I will update the known issues section of the roo reference accordingly.
I am closing this ticket as 'won't fix' for the reasons discussed here.