At least this is true for two phones that we have - they've been bought a month apart (October and November 2008, respectively) - and the battery life has dropped sharply from being able to normally use the phone all day (from eight in the morning to midnight) to being dead at teatime, within a few days.
Prediction: we're about to see a rapid increase in battery life complaints, sharp rise in G1 battery demand, and, as a consequence, sharp rise in the price.
Go get yours before it's too late.
UPDATE: Don't be stupid and try to buy the battery from T-Mobile for $49.99 plus tax. FIYF, I'd say I'll be comfortable with anything above $5.
UPDATE: Got the replacement battery (identical to the original). When I opened the cover to install the replacement, I found the original battery swelled to about 120% of its initial thickness. Makes me wonder how far I was from a battery fire - I remember the phone getting unusually hot when recharging lately.