I'm a programmer on the ACS-Java project, I'll try to answer as best I can. First off, JSPs *are* servlets. JSP is a scripting language that the servlet engine compiles down to a Java servlet on the fly. So, if you have something that can handle JSPs, odds are quite good that it can handle servlets. The only additional issue with servlets over JSPs is generally a configuration issue; servlets that aren't JSPs usually need to be registered (in an XML init file) for the servlet engine to locate them. I suspect this is the step you've been missing with your nstomcat experimentation; you want to edit Tomcat's server.xml file to map a context path to a docbase, and then edit that docbase's WEB-INF/web.xml file to register servlets. Unfortunately I haven't played with nstomcat any; in the interest of just working on the ACS as much as possible, I went with the quick "run Tomcat stand-alone" approach rather than trying to configure a webserver to talk to Tomcat.
The ACS has used Java in Oracle for some features before, even back with the TCL version. That's the "java in Oracle" you've seen. For ACS Java, though, the code written is outside of Oracle; it runs in the servlet engine's JVM.
ACS Java does not require AOLServer or Apache. It requires a servlet engine; various servlet engines can be made to work with various web servers, or can be run stand-alone. We've run under Tomcat, Resin, and WebLogic servlet engines. Adding a webserver is essentially using the webserver as a proxy engine, where it takes web requests and hands the requests for servlets (or JSPs) off to the servlet engine. Again, I haven't found a need to personally experiment with this sort of thing yet, so I don't know much about it.
In response to Kapil's observations, yes, ACS4 Java is quite similar to ACS4 TCL because it was a port, not a redesign. ACS5 will be a redesign, and thus will end up being different, much like how ACS4 was a redesign and thus was very different from ACS3.
I'm not sure what openacs will choose to do with regard to ACS/Java or acs5; I suspect it's quite quite early to speculate on such things, and that decisions will be based on technical merits, business needs, and who has free time. I'm not involved with openACS, but I like the project, read the bboards, and will be using the code for a personal site as soon as I find free time.
Hope that clears things up a bit.