I want everyone to have a default membership level (defined) and not show a drop down to select.

Mt little comment trick posted above didn't work so good in 4.0.5.

Any suggestions?

havent tried this is 4.0.5 yet but add a customer with the membership level you want, check with phpmyadmin what the level code is (in 3.x version it was C i think) go into register.tpl change the field to a hidden field and then set the default value as C