D.C. United qualified because of their performance in the old CONCACAF Champion's Cup, not because it's from United States(just like Saprissa and Motagua got to play it too) also it's likely that United States won't be invited to Copa América, also what Naui says is true, and so far as i know it was because CONCACAF didn't allow them to send the best performers of the season to a non-CONCACAF tournament, let's see what happens, CONCACAF may have something to say too, and they may see as the best oppoturnity.
travel is just too far, for MLS to go to libertadores. The only thing that could happen could be in the late stages...such as concacaf teams joining in the final 16 or 8 or something like that. Playing week-in week-out in the group stage just wont work. Obviously, conmebol isnt just going to let concacaf teams join that late unless a whole lot of money and exposure it at stake. I could see something like conmebol leaving 5 or 6 spots of final 16 for top concacaf championship teams if MLS teams or FMF teams had names like Tevez, Diego, Pato, Malouda, etc, etc. When that happens, conmebol will come calling.