To be sure everyone understands what debdelta does, if you're not already; please have a look at this site and the readme files and other information there:http://debdelta.debian.net/
Basically, as a high-level, and somewhat, apologetically, pedantic explanation, let's say you found you needed to upgrade a package which is 4.5 MB in size. Currently, without using debdelta, the package that you have on your computer would be deleted, and replaced by the new package, necessitating a 4.5 MB download. Debdelta can be your friend, as it will do a comparison of what you have on your computer with what is contained in the new package, and only download the difference between the two packages. There may only be a 1 MB change from what you have, and that's all you would need to download. Debdelta does all of the comparisons (runs a diff routine) and then provides you with, ideally, the same result as if you had downloaded the entire 4.5 MB. Now, figure the savings in download time that could result if you are doing a large upgrade, with many packages. That's the beauty of debdelta. It saves both server time, and your download time; especially valuable for those who have slow connections....and, more on the actual topic...I apologize for that, but I thought it worth the explanation given the stabilization of Squeeze.
Now, assuming you've kept up to date, and basically have a completely updated Squeeze system at this point, but have elected to continue testing. As packages are updated and released, debdelta will, when evoked from the command line, compare what you have on your system, with what you will be replacing that with, and download and install only the difference.
That's my understanding of it, and I hope I've not misinterpreted anything, nor misled anybody. Please correct me if I have.