I haven't really heard of any such constraint on Struts 1.1 due to the usage of Tomcat 5.x [though I've been working on Struts 1.2 for a *long* time now].
The only problem I can think of is:
a => The Tomcat 5.x container requires that your web app have a more recent DTD version for the web.xml, which in turn requires a newer Struts DTD (= a newer Struts version)
b => The Struts source code for 1.1 might be based on a JDK version which the Tomcat 5.x container isn't too comfortable handling. [<= a bit improbable though 'coz the JDK versions are s'posed to be backward compliant]
IMHO,
you should consider migrating to a more recent Struts version. At most, you'll only need to change the struts-config DTD and change some tags here and there. Isn't a big overhead at all.
As for learning about Struts, I haven't got much clue about the Dummies book but you might want to take a look at this
one.