The LiveUSB HOWTO (http://www.gentoo.org/doc/en/liveusb.xml) refers to the "dobladecenter" boot option; apparently this was renamed to "doslowusb" as on Gentoo 2006.1. Other HOWTOs etc. may also be affected.
The LiveUSB guide is specifically for the 2006.0 stages, as stated in the examples. Do not attempt to use a 2006.1 stage (or its commands/boot options) with this guide.
Sorry, but it does not state that the instructions apply only to 2006.0. If that is your intention, then it should be explicitly stated - not just implied: rename it to "Gentoo Linux 2006.0 LiveUSB HOWTO", have an "Important" or "Warning" box stating it does not apply to 2006.1 etc. Also, as stated in the original bug report, other documents still refer to "dobladecenter" e.g. the 2006.1 Installation Handbooks. These need to be changed to "doslowusb".
Fixed and fixed in CVS.