Actually, if a Steam game doesn't specifically have CEG, even if it's permanently tied to the client itself and an account that purchased it (which doesn't actually guarantee that CEG is involved), there's a way to run that game forever, offline, without any online checks or downloading any cracked files, on any number of PCs you choose. That's a completely different thread for another time, though... I've been meaning to start a thread here showing people how to do that, actually.
Very interesting. I would definitely be interested in knowing more about your methods, and whether they are fully legal or allowed by Steam's terms of service.
Another though occurred to me, how reliable is Steam's offline mode? Is it possible to just clone a full system hard drive as a backup and then forever be able to use that system drive to play Steam games without ever connecting to the internet?
You actually don't even need to do that. If you start a game online once, it downloads everything it needs license-wise into the Steam directory itself, and Steam is a completely portable program. All you need to do is download a game, start the game online once, put Steam into Offline Mode, and exit the client. Everything can literally be copied/pasted to as many PCs as you want to now, and you'll be playing in your own offline profile every time. Offline Mode will work forever without any restrictions, and since you're using Steam exactly as intended, I can't see any way in which it would be against the law or any part of the SSA. I've moved my main profile and games to PCs that have never once had Steam on them, and everything always worked perfectly.
The write-up I want to make details the difference between regular Steam client DRM and CEG, since there is a big difference, and how to tell which games have CEG (i.e., the ones that can't be transfered via this method) and the ones that don't.
As far as cloning a hard drive, CEG games still won't work, because CEG actually locks games to the original hardware onto which they were downloaded, until you go online and verify the cache to get another hardware-specific program file. Yes, you read that correctly: Valve, the company that takes every chance it can to crap all over companies that use DRM, the company that requires third-party DRM to be listed on every Steam game page that uses it, created a DRM that actually locks games to specific hardware without asking permission to play them again.