I'm looking too, but for the [Doubleshot].
Trying the same thing myself on the HTC Doubleshot (MyTouch 4G Slide) and no joy, same exact error.
ericosman, did you by chance follow all the advice on HTC's site -- including specifically installing HTC Sync? I didn't install Sync because it looks like stuff I won't use and don't need, except that Sync has some dependencies (like a USB BMP Driver, whatever that is) that might make the difference. I haven't tried yet but I plan to.
I've tried two different versions of Fastboot, one I downloaded from masterjason.com and another I downloaded from Google's Android SDK manager. Both behave identically, despite one being nearly a megabyte where the other was 70 kilobytes.
I've tried reinstalling USB drivers, no joy.
I can get fastboot devices to work, as well as adb anything when the phone is booted, so I don't think drivers are really the issue.
I think it's HTC making a blanket promise that all the phones in that list can do this and indeed not all can.
[EDIT 2012-01-07: Add more info I found]
Gotta love it. Edited the post, and when saving, was told to refresh and log in. Then I wasn't given a chance to copy-paste what I typed. Annoying.
Anyway, see if I can remember what all I typed...
I found through Google, and directed to Yet Another Android Forum somewhere on the 'net, that fastboot oem h will provide you a list of commands that are supported, and get_identifier_token was not among them. I suspect it's the phone's bootloader which contains supported commands, not Fastboot executable itself, so it may require downgrading/upgrading to a different HBOOT that has the necessary command support.
I did find readsecureflag which returns the integer "3" on my S-ON device. I assume then that the command I found called writesecureflag similarly takes an integer value, but what value to feed it, I don't know, and I don't want to risk my phone. Seems benign enough, but I know from computer programming that anything looking benign is not always.
Anyway, if you run fastboot oem h and get_identifier_token isn't there, I'd recommend giving up on this particular method of rooting.
Hope this helps.
[Edit 2012-04-15: Strikeout above no longer applicable info]
Turns out, see Aisho's post below, which sums up what eventually worked for me.
I actually had to wait for an OTA to be available long enough for a dev to package it up to a bootloader-flashable package, which included the get_identifier_token. Once I had that version on my phone, I was able to get the token to HTCDEV and get my bootloader unlocked. Not full access, but not bad: The only downside is if I need to flash a kernel I have to do it separately from the ROM through fastboot, and I don't think I can update my radio (not that I should need to). But hey, I can flash a custom ROM and get rid of this nasty crap T-Mobile thinks everyone should have (I will never play bejeweled, so I don't even want that adware/nagware on my phone).