A sub is non directional, and as such, radiates sound in all directions. Confining it in an enclosure will probably not give you an optimal result. In fact, just it's distance to the nearest wall will have a big change on how it sounds.
To go even further, the general placement in the room itself is equally important and some people spend a lot of time moving the sub around the room finding the optimal spot for the sub so that it is least effected by nulls in the room that are caused by reflections canceling out the big lazy sound waves of the lower frequencies. For instance, in my small square room, if I place my sub between my TV and my right speaker, I get bass in the corners of the room loud enough to make your shirt shake but sitting on the couch I get very disappointingly weak bass. Simply moving it about 4 feet over to the right and a bit closer to the rear wall so that it is closer to a corner smoothed out the bass in the room so it is much better in my actual seating position. The difference can be very dramatic.
Personally, it is not something I would do but sometimes you have to do what you must.
You can purchase subs designed to work in a wall or an enclosure as you describe. However, the 350 is a great sub and I bet when properly placed, will sound better than most wall units. Why waste it unless you really need to reclaim that bit of space?
Edited by Murph (08/24/10 07:25 AM)
Edit Reason: added thought in in-wall subs
I'm Riffing. People usually stop me when I'm riffing. Or carry on without me. That's also an option.