If it was true then, it will still be true now. Patents don't expire quickly. I've never heard of microsoft ever enforcing this particular patent though...
The core patent claim is:
A computer implemented method of providing varying levels of detail in terrain rendering, the method comprising: providing representations of plural levels of detail of a terrain, where each level represents a regular grid; determining a viewpoint of a viewer; caching a terrain view in a set of nested regular grids obtained from the plural levels as a function of distance from the viewpoint; and incrementally refilling the set of nested regular grids as the viewpoint moves relative to the terrain.
Or in english:
It's a terrain LOD system. Each LOD is a regular grid (evenly spaced vertices). The LOD selection is based on the camera position -- depending on the distance from the camera, a different representation of the terrain is generated. These generated LODs are cached in this nested regular-grid structure.
By the sounds of that, if you implement it using texture-fetching in the vertex shader, you wouldn't be caching the grid structure, so you wouldn't be infringing the claim.
Or, if you use nested circles instead of nested grids... or if your grids weren't regular (e.g. if the vertex spacing gradually got farther apart as you went outwards on the rings).
So it's probably pretty easy to implement clipmaps without actually infringing that patent... and even if they did sue you over it (which they've not done to anyone yet, AFAIK), if you had the money to hire a legal team, you could probably argue that a "nested grid comprised of texture images" is an obvious analogy to the mipmap, so that their claim isn't even valid...
As mentioned in that other thread, it's probably a "defensive patent", where they're not looking to enforce it, unless someone tries to sue them for patent infringement first -- in which case they'll counter attack with 1000 stupidly obvious patents of their own. I bet you use a linked list somewhere, or have a chat system that allows people to send strings of characters that look like faces (":-)") to each other, for example...
The usual disclaimer: I am not a lawyer.