I had the same problem and how I fixed it was I reinstalled liberty and used the liberty toolbox to change it back to the 'M' then just used bootstrap to revert back.
I have a d2 but I don't see y it wouldn't work with a dx.
@ both having this issue. Just switch it out with a different image. Ive made a couple hundred over in the hack section. The original M is also in there. Page 46 I believe. The thread is called "custom droid x boot logo, I'll create."
The Liberty Toolbox will still work on other rom's for certain things (Build Prop Edits, Boot Logo, etc) just redownload the Toolbox and change the boot logo to motorola.. don't try any themes though