Application Impact Management is a fancy name for the virtual sandbox that Microsoft is creating for applications that attempt to use resources that the limited user account (LUA) doesn't have permissions to use.
For example, if an application running as LUA attempts to write HKEY_LOCAL_MACHINE or the Program Files, Longhorn will create a virtual copy of that resource for the user and give the program access to that resource. Obviously this isn't a panacea for LUA bugs, as this creates its own set of problems. One should probably assume that if a program is writing to an "all user" resource, it is doing so for a reason - although this is certainly not always the case.
The AIM technique does however solve one of the most significant problems of "RunAs"-based workarounds (because it doesn't change the identity of the user). RunAs is a tool (and actually has become a set of techniques) that attempt to resolve the restricted resource issue by running an application under a different account, with access to the desired resources. However RunAs solutions change the identity of the user and expose the credentials of the elevated account to the end-user. Creating files or accessing resources under different credentials creates a host of problems that are collectively known as the "RunAs" problem.