I wanted to store a link to Alastair Bridgewater's (a.k.a. nyef on IRC) SBCL-based LispOS notes.
Nyef's most apparent distinguishing characteristic is an amazing willingness to hack low-level internals on Intel x86-based Lisp implementations. Given the claim that "The classic problem with the 'LispOS Project' is that a large portion of the early effort would involve low level hardware hacking", nyef seems to be the ideal candidate for overcoming that problem.
Also, just wanted to record a link to a vendor selling the remnants of the Interlisp environment. Seems like for something under US$3000, one can purchase a x86-Linux-compatible version of the Interlisp programming environment.