I've seen other vB forums with the sub-forums (like microcontroller->8051 and member's lounge->introduce yourself) listed right on the main page with links under the parent forum.
Yes it's definitely possible and we actually had it setup like this until recently. I have chosen to hide them to un-clutter the front page (especially for new members coming to the forum).
It is also most likely that when they go to post in one of the main categories (ie Microcontrollers) they will see the sub forum (ie 8051) when entering into it as they are quite prominently placed.