Spring OSGi and Impala are tackling an overlapping set of problems
Impala provides a developer productivity solution through build support, an interactive test runner, and support for efficient, fast running integration tests.
OSGi, on the other hand, tackles a wider set of problems relating to visibility of classes. OSGi allows you to run multiple versions of third party libraries within the same JVM, which can prevent problems which may occur from different libraries depending on incompatible versions of the same third party library. You can think of OSGi as defining multiple class spaces for third party libraries as well as application code.
Impala makes a clear distinction between application code and third party libraries. There is only a single class space for third party libraries. If there is a clash between third party libraries, then you are no better off (or worse off, for that matter) than you would be in a standard Java/pre-OSGi world. You would still need to resort to whatever workaround would apply in that case.
While third party library clashes are certainly a theoretical possibility, it is quite rare in my experience to actually be a victim of such problems in a way which couldn't be addressed relatively easily through a simple workaround. Others may have different experiences in this regard. However, it does seem that there is quite a bit of overhead required to make sure that the Jars you use fit in nicely with OSGi requirements, so that they "play nicely" in OSGi world. It's the kind of overhead that I think most developers would look for ways to avoid for as long as they are able to do.
The purpose of Spring OSGi seems primarily to bring the benefits of OSGi to Java, via Spring. In that sense, it is OSGi centric. The fundamental trade-off with OSGi is one of productivity. Are you prepared to go to quite a lot more trouble in defining quite precisely the nature of dependencies between libraries, with the benefit that you will never get ClassCastExceptions due to classloader issues, and that you will be able to dynamically update library versions. Make no mistake, these benefits come at a cost. One very experienced OSGi developer described working with library dependencies OSGi as a "pain in the arse", something you wouldn't want to do without tool support.
Impala, by contrast, does not impose these kinds of constraints. In fact, it's very essence is about productivity. Dynamic module reloading can benefit productivity, because it means that integration tests can be written on the fly against a running application.
Impala Support for OSGi
Impala has all the internal interfaces necessary for seamlessly supporting OSGi, probably via Spring OSGi. I'm definitely considering adding this in the future. For the time being, that's not on the immediate horizon - getting the project into a publicly releasable form is a higher priority right now.