[plt-scheme] sxml package runs away on load, but only in mzscheme

I'm observing something strange about the new sxml/ssax packages;  
requiring the package in mzscheme hangs and consumes all CPU, but  
requiring it in DrScheme does not.  Also, the first download from the  
planet repository also hangs.

How to reproduce:

1) Start mzscheme (v3.99.0.9)
2) evaluate (require (planet "sxml.ss" ("lizorkin" "sxml.plt" 2 0)))
3) wait until you're convinced.
4) hit CTRL-C.

How to anti-reproduce:

Try the same thing in DrScheme, in the mzscheme language level or in  
the module level with the program

#lang scheme/base

(require (planet "sxml.ss" ("lizorkin" "sxml.plt" 2 0)))


... I'm not sure what could cause this kind of error, though I bet   
errortrace would tell you.

John Clements