I've looked into this too and found pretty much the same thing. All will insure for 12 months, some for 18 months, very few for upto 2 years and non for longer than that. One company told me it was to stop people who are living abroad but still registered in their home country from getting round expencive medical insurance and other normal resident insurances by using travel insurance.
Sounds reasonable I guess. All the company could suggest to me (can't remember which company it was) was to fly home to get your passport stamped and then fly out again. This way you qualify as having started a new trip - expensive way of doing it though.
I asked what happens if you just say you've just started your trip without actually going home to get round this. The simple answer was that if you do claim then they check your passport. Ain't worth the risk then IMO.
Grant - It would be good if you could put this in the e-zine. It seems to be a bit of a problem for people travelling for more than 18 month/2years.