Running a OSGi framework inside other containers is doable (but as others have noted some are easier than others - eg.
servlets easy, ejb tricky). The footprint of for example Apache Felix is about 500K (both on disk as well as in memory) so typically no problem for a server side application.
As mentioned already, more and more, there are possibilities to use OSGi out-of-the-box inside JEE containers (JOnAS has very good support but, Glassfish, JBoss, and Websphere make it possible one way or the other by now as well).