youll need to be on current firmware(at least the radio and bootloader) to get the hboot to give you an unlock token.
i would advise against doing this,since the eris only has s-off with an eng hboot. ive unlocked and used the htcdev hboot on several s-off phones,and have messed with it quite a bit on my rezound.
a big prollem with htcdev on other devices is that you cannot write to the boot partition from recovery. in order to flash new roms,you must flash the rom in recovery,and if it contains a different kernel that is neccessary,you must seperately flash that in fastboot.
its certainly better than nothing,but compared to s-off,or even "normal" s-on flashing on the eris,its a PITA.
i could be wrong... it may be that unlocked s-on flashing will work the same as it does with 1.47 or 1.49 hboot s-on,but if its not...
be aware that you may be stuck with that hboot forever
the cdma evo 3d guys still have not found a way to downgrade back to s-offable software,and my experiences with it on the incredible were that it remained in place even after downgrading to eclair wich uses a completely different hboot than froyo or GB. we were lucky on the incredible and got an older version of unrevoked to turn the secureflag off with the htcdev
locked hboot in place. with the eris this wont be an option,and even changing the misc image and flashing the root rom may not bring the 1.49.2000 eng hboot back.
ive run htcdev on my s off merge,inc,wildfire,inspire and incredible 2. when combined with s-off,there is benefits of it since gives extra fastboot commands. sort of a "half-engineering" bootloader
i wouldnt risk it,tho,without a gameplan to get rid of it if you find boot is no longer writable via recovery. all the crazy stuff yall have seen me do to my lil buddy,no way ill flash an htcdev hboot on him!
may want to look thru this thread before continuing to try and get htcdev to work:
http://androidforums.com/incredible-all-things-root/495687-htc-dev-unlock-problems.html we dowgraded from GB to froyo,and then from GB to eclair,and the 1.02.0000 hboot would not allow itself to be replaced with the older hboots
my 2 cents,anyway,for what theyre worth