So...finally...WPMU and WP Ecommerce work seemlessly together. I have tested this extensively and can happily say all is well in the world.
For my tests, I installed WPMU 2.8.4a and the newest Development Version of WP ECommerce found here:
http://wordpress.org/extend/plugins/wp-e-commerce/download/
After installation, I tested on the main blog and everything works great. I then set up a user blog and again, everything works great...no "these directories are not writeable, etc"...it just works.
So...if you're in the market for a WPMU/WP Ecommerce setup...you're in business!
http://www.shaynesanderson.com

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