Well, I have yet to see it happen in SOF2, but in RA3 it goes like this.
Team A loses a player mid round, times out or whatever. So they have to play that round down a player, sucks, but it happens. After that round is over, there is a message that pops up, something to the effect of "Teams are uneven". The only way the next round will start is if Team A gets another player, or Team B drops a player.
RA3 will not let a round begin if the teams are uneven, in competition mode.
Not sure if SOF2 is like that or not. Actually, I dont think it is. I think Asstanx just told me that they played against a team last night 5v4.