After you learn five things about Aaron that you didn't know, he'll show you how to do an incremental get without necessarily doing an incremental build.
In Team Build v1.0, it is not possible, without some trickeration, to do an incremental get without also doing an incremental build. (The process for doing both is documented here, and consists of setting the SkipClean property to true, the SkipInitializeWorkspace property to true, and the ForceGet property to false)
The short answer for why this is the case is that the CoreClean target wipes out the entire source code directory in order to delete the intermediate binaries...
Next he shows you how to get a list of files that changed since the last build.
I've had several people inquire recently about how to figure out which files have changed since the previous Team Build build... In V1 there is no straightforward way to do this, unfortunately, though we plan to remedy that in the next version of Team Build. For now, here is one approach - the basic idea is to do a preview get prior to doing the actual get. There are issues with this, of course - you might be interested in the list of changesets rather than the list of files, for example; additional modifications might be made to source control between the preview and the actual get; you might want to get one of these lists without having to do an incremental get; and so forth. To a first approximation, however, this should do the trick.
As Aaron states, we're working to make these and other things simpler to accomplish in Orcas. These are the kinds of things you won't find on the product roadmap or in a bulleted list of product features, but they help make folks more productive. We take the feedback from the build forum to heart and try to address as many of these things as possible. Whenever you hit something that's not possible, painful, or just simply annoying, post it in the forum. We'll do our best to either make the problem disappear or at least substantially improve it.