OK, I first went to their warranty page, and was unable to find a policy regarding battery. I have emailed HTC asking what they would do should the battery faily to hold a significant charge at 6 months, or 18 months, and am awaiting their reply. Once I get that reply, I will post it here, and then we'll all know for sure what their position on battery replacement is.
In all likelyhood the battery will easily last you more than 2 years, and should still give good life at 3 years. My concern is though that is usually the case - what recourse do I have if I get a battery that is a lemon? I'll post a reply once they reply to me.
Here is the warranty I was able to pull for their Android phones in the US:
http://dl4.htc.com/Web_materials/Manual/Warranty/Android/0525_G_WarrantyEULA_76x126_US-BP.pdf
Their limited warranty is for 12 months, but nothing in the warranty talks specifically about the battery, except voiding the warranty by shorting the battery or other mallicious means.
It would be a shame if the warranty for the battery was only 12 months, when most people are on a 24 month contract. As badly as I want this phone, It also makes me hesitant knowing that I have no recourse should the battery fail at 18 months, which is even more punishing because verizon/att are scrapping their 20 month upgrade renewals and pushing it even further to 24 months.