There is a limited form of eminent domain which applies to government use of patents.
 See: http://inventors.about.com/library/bl/toc/bl_patent-infringement.htm
  A patent holder cannot sue the government or its contractors for infringing a patent. The government  takes a license, determines a fee, and pays it.

 A (far fetched) application of this could be that the government could contract the development of an anti-spam tool incorporating M$ Sender ID, pay M$ a fee it deems fair, and make it available to the public. Don't look for this any time soon...

 One thing you can't do with real estate is buy up all the property around someone else's  property and then deny or charge for access to that property. In some cases, this seems to be the primary purpose to Intellectually Property.*  

 A much more desirable avenue would be to rewrite the statute to allow the flat out taking of a patent for the general good, or a forced, royalty free license for a specific use determined to be in the public interest. And (duh) striking software patents as a really stupid idea.

 Real estate is generally subject to taxation based on an assessed value. Patents and copyrights are not. Only the income from patents and copyrights is taxed.  However, hoarding copyrights and building patent warchests has economic value separate from any actual or potential revenue.

 Patent warchests serve the same purpose as nuclear arsenals in the cold war. Attack us and we can nuke you. If you have your own nuclear arsenal, then there is a standoff based on MAD (Mutually Assured Destruction). The value in this for the arsenal builders, as Bill Gates kindly pointed out, is that it raises the barriers to entry. You end up with an exclusive club of patent superpowers, any small fry that try to get in are nuked or eaten.  Unfortunately, as in the political world, terrorist organizations can aquire these weapons and wreak havoc. In the IP world, these terrorist organizations are known as patent pools. They have no business to be threatened with retaliation. Worse, they can be used as proxies, circurmventing anti-trust laws and cross-licensing treaties. Some of these patent pools are the remnants of legitimate concerns who have been either failed to successfully compete in the market place or been deliberately crushed by the big palyers (is that an artificial distinction?). Others are simply bottomfeeders out to extort cash from legitimate concerns by any means.

 Copyright hoards serve several purposes. One is that the holders, by keeping the copyrighted material out of circulation, avoid competing with their past successes. It is easier to promote new proprietary high priced junk if it does not have to compete with inexpensive quality material. Also, nearly everything written since the creation of Mickey Mouse is under copyright, assuring that just about anything you write has the potential of infringing something you have never heard of. Copyright hoards are not in the public interest. Pay taxes on them, or they become public domain. We DO have a deficit to consider, ya know? Yeah, ain't gonna happen.

 <digression> I notice that some groups hold property rights to be sacred and the whole concept of eminent domain to be sacrilege and abomination...despite the fact that all land "title" the world over generally goes back to a point where the original occupants were killed or driven off. </digression>