Versions 3 & 4 of our API were compressed yesterday - they went out with the release of the new version of Windows Live Local/Maps, Live Maps, Live Search Maps or choose your favorite name, but remember Virtual Earth is the platform and not the web site.
Anyway, versions 3 & 4 were considerably reduced in size: Version 3 was reduced from 338k to 92k and Version 4 was reduced from 442k to 104k. You should see performance increases in your application if you're using either of these controls. Also, since it was done yesterday and my phone and inbox haven't been blowing up, I'm going to assume the upgrade was seamless and there were no breaking issues. There could be potential issues if you downloaded the control and modified it - not a best practice and against recommended guidelines - but, if you were brave enough to do that you probably have a contigency plan in place for when we make changes.