[Edsquire]

If we're talking about an invention covered by a currently active patent, like electronics hardware, the rules about selling the clone change (can't do it) but there's still nothing stopping me from building a few copies for my own personal use.

Not true.

35 U.S.C. § 271 (emphasis added):
(a) [W]hosoever without authority makes, uses, offers to sell, or sells any patented invention [in the U.S., or imports it], infringes the patent.

[Arthalion]

Not true.
35 U.S.C. § 271 (emphasis added): (a) [W]hosoever without authority makes, uses, offers to sell, or sells any patented invention [in the U.S., or imports it], infringes the patent.

Maybe so, but courts have somewhat consistently (though not exclusively) supported the concept of "experimental use" by falling back on the legal theory for the granting of patents...that they should allow inventors to profit from their inventions without having competitors steal their markets. Courts have held that "experimental users", people who duplicate a patented object for their own use and enjoyment without the intention to sell or profit from the resulting duplication, are not infringing on the patent holders ability to market and profit from his invention, and are therefore exempt from patent law. If I made one or two in my garage for my own use, it's legal. If I made 50 and gave them to my friends, it wouldn't be. If I made one or two copies of this companies woodworking equipment for my business, with the intent to profit from its use, it WOULDN'T be legal. If I were a hobbiest and built my own copy of this tool so that I could build myself a better gunrack for my family room, it WOULD be legal.

One of the problems with patent law is that it isn't as clear as it looks. The law itself is simple, but the tremendous amount of case-law that exists on top of that law to "define" its meaning can make actual legal practice both confusing and contradictory.