Your source of information concerning the fact that Droid X's bootloader is signed? I'd tend to agree that it is, but I haven't seen anything official.
While the Milestone does have a signed bootloader, that doesn't mean there won't be a work around hack found. It hasn't happened yet (going on 7 mo since the phone was released), but the phone
has been rooted, and the hackers do expect to find a solution to the signed boot loader. Check out
this site for more information.
So the Droid X should still be able to be rooted - the Milestone was within about a month of release. This will allow users to add programs, overclock, tether, etc since you gain control off the low level functions of the phone.
All that being said, I've gone from being very excited about the Droid X to deciding I'm waiting to see what happens on the root/bootloader front before I jump in and buy one. If it does end up being impossible to crack the bootloader, then I will wait for another "killer" phone to buy - I doubt I will have to wait too long.