Previously limited to 50MB APK files alone, Google is now allowing direct downloads of up to 4GB in extra resources (two files, each up to 2GB) for apps obtained from the Android Market, a capability first announced at Google I/O last year. In practice, that means that developers will be able to push large apps entirely from the Market, rather than putting users through a two-phase process where they download the application first then download a significant amount of additional data the first time the app is opened.
This won't have much effect on productivity apps, utilities, and the like, but it's big news for Android games — particularly graphics-heavy 3D titles that nearly always require a long download process on the first start. For users, the move also means that they'll be able to see the "true" size of applications that they're downloading from the Market, which can be significantly higher than the APK alone. Additionally, the 15-minute refund period won't begin until the entire app is downloaded; currently, it's not unusual for the window to expire before you've even had a chance to install all of a game's extra data.
Google says that "most newer devices" will be able to take advantage of the capability, but some older models will be able to hook into a service that downloads the resources directly from the Market the first time the app launches — in other words, similar to the old behavior, but you're grabbing the data from Google's servers rather than a third party.