i think froyo will be less battery consuming. coz i have a n1 with 3G turn on whole day vs my gf desire 2.1 without data plan means she turn off 3G the whole time. but both of us get roughly the same use time with 1 charge. not to mention i also play with my phone more if compare to hers... hope this help...

will need some time to see whether desire got any battery improvement after update to froyo. go go FROYO