There isn't any. It's voluntary and nothing more than a goal. No enforcement, no guarantee, just a stated goal. Also, you have to remember that our phone was in development well before this goal was ever publicly stated. The things that would help with meeting this 18-month goal of support (like more RAM, more space for system files, etc) need to be known and incorporated during the development phase. There's no reason to think this was the case.
A new car that is introduced at the same time as new federal safety regulations will not be held to those standards, because the car has already been created. The regulations are for governing future development, not things that already exist.
My assumption is that manufacturers, going forward from that agreement, would try and make sure their hardware was sufficient to support at least 18 months of software upgrades and that their UIs were internally consistent enough to make the upgrades easier and quicker to deploy.
Why do you suppose that car manufacturers aren't held to those safety regulations? I'd argue that its safe to say its because they can't adhere to a regulation that doesn't exist yet, right?
If that's true, and the phone manufacturers before were only capable of creating devices that have "relevant" hardware 6 months from production, why are we to believe that they're now capable of seeing what software demands are going to be over a year out? How can they hold themselves to a standard that doesn't yet exist?
To ask another way, look at one of your own sentences;
"(like more RAM, more space for system files, etc) need to be known and incorporated during the development phase"
Short of a crystal ball, how will they know these things that "need to be known during the development phase" if the software hasn't been developed yet?
Conversely, do you think that they were limiting the hardware they were putting in phones to only minimally meet the software demands of their current OS? I don't, as the progression we've seen in hardware over the lifespan of gingerbread has increased pretty significantly, which would lead one to believe that hardware development (and not simply meeting minimal software requirements) is what is driving what is actually making it into the devices.
By your framework, they're either incapable of making a phone relevant for more than six months or they've been intentionally not doing it. I don't see (based on technological progression) how anyone could argue the latter, which would leave only the former. If the former is true, why would they be able to TRIPLE that time frame now? You think they weren't "trying really, really hard" before? Or do you think they'll be able to predict the future and know what the software reqs for the next major iteration of Android will be and are focusing their development on that?
Does that mean that the 18 month thing is just a sham and isn't even a realistic goal to be set?
How should I feel about the Thunderbolt getting ICS? It was released even earlier than the Charge. Is that an indictment against Samsung? I'm genuinely asking...not trying to be combative.