Well at 1.8GHz it has a power dissipation of 42W -- 12W higher than a 1GHz G4. While this is pretty good for how much of a speed increase it is, it is still more. As for getting it in to the iMac, how long was it between the introduction of the G4 and its inclusion in an iMac or eMac? I think that based on what we're seeing coming out of Motorola right now combined with Apple's general pattern of behaviour the iMac will continue to be G4 powered for quite a while to come.
Regarding that 1.8GHz AMD system you built, did you squeeze it in to a case the size of the iMac? Was there an LCD floating overhead on a metal boom? Did it have low power consumption and only one, practically silent fan? No? Maybe that's because it's a different computer. Yes, anyone who compares the two is going to find that the peecee is faster. But anyone who absolutely must have the fastest computer and values that aspect above all others is going to go for the AMD or Intel powered machine anyway. Most people who buy Macs aren't doing so because they unconditionally trust in the MHz myth and simply can't believe than anything could be faster than a Mac. They are doing it for other reasons entirely. I won't bother trying to enumerate all of the reasons since I'm sure we all know them. Suffice it to say that there is more on the side of the Mac than processor speed.
So I don't think it is greatly important for the G5 to make it into the iMac within the next six or even twelve months. I don't think Apple will keel over dead if the iMac is still G4 powered in eighteen months. I think the places we need to see the G5 are in the Powerbook and the Xserve. And for that I am willing to pay a premium.