The Walmart Canada plug-in is now compatible with Lightroom 3 Beta:

Export to Walmart Photo Centre (Canada): 20100125.2

Also, now the plug-in correctly handles the situation when you attempt to upload photos that exceed the 6MB size limit of Walmart Photo Centre.

A note for LR3 Beta users: due to some internal changes, Lightroom 3 Beta no longer displays the branding graphics at the top of the export dialog.