This is a terrific question. You have hit on two of the most important things people ask me about. One is: should you take a job in an area of SAP you might not prefer because that is where the opportunity is? The other is: can you get "trapped" in ABAP?
Let's look at the first part of the question: ordinarily, when you can find a company to train you in SAP, that is a pretty good deal. Even for promising young college grads, it can be hard to find a job that trains in SAP. In these kinds of situations, it all depends on what other job offers you have in your hand. In your case, so far, you only have one. What I would do is to postpone accepting this first job offer as long as you reasonably can without losing the opportunity. In the meantime, apply for some other kinds of SAP positions and also some non-SAP positions in Java and C++. See what kinds of job offers you get, and then choose the best one. Let's assume just for the sake of your question that you only end up with this one offer: is it smart to take an ABAP job, or are you really "trapped in ABAP?"
I don't believe you are trapped in ABAP, and here's why: SAP has built support for ABAP into its NetWeaver architecture (consider the NetWeaver Application Server with ABAP, for example. SAP even has it own version of Web Dynpro for ABAP. But most NetWeaver components are "wrapped" in layers of open standard languages such as Java and XML. So this means that an ABAP person on a NetWeaver project should also be able to get exposure to web-based languages and integration tools. As long as you can find your way into these kinds of forward-thinking projects, you shouldn't get "trapped" in ABAP. On the other hand, I do think that ABAP is subject to more pressure from global offshoring than other areas of SAP. This means more competition for openings and lower rates.
On the other hand, many Java and C++ programmers are in the same boat. I think you could say that all kinds of programmers face similar challenges from offshoring, so those issues are not unique to ABAP. ABAP is not the most marketable area in SAP, but there are definitely some ABAP programmers doing very well right now while maintaining exposure to Java-related technologies. I write a lot about the challenges facing ABAP programmers in this column, so keep reading.