My first thought was yes, but after some thought I agree with Skylancer4. In real life, where we don't have to wait our turn to act there would be no reason for someone to "bullrush" you if you were going to let them do it unless you were immobilized.
If you were engaged in a melee with an enemy and then your ally calls out "I'm going to run up and push you out of harm's way. Hold Still!" and then you stopped dodging your enemy and let your body go limp when your ally hit you so he could easily move you, you would most likely have time to just run out of the way yourself (if you didn't have to wait for your turn).
In the game, which uses turns to make things manageable, you generally don't get to do even simple things when someone else is taking a turn except a few immediate actions. Just as going on "Full Defense" requires an action, relaxing your defense should also take an action on your turn so that if you are letting yourself be pushed by your ally then you also are not doing your full defense against any enemies so your CMD and AC would be lower for the whole round. I would also have to consider whether doing this provokes an AoO from threatening enemies because you are not applying your full attention to defense, or at least makes you vulnerable to sneak attacks because you are not using your dexterity that round.
And furthermore, consider mounted combat. If you are riding a mount and it moves you, you are only able to make a single attack. Allowing yourself to be moved is the same as moving.
Let's say an enemy is 15 feet away from you. Your ally standing behind you has the turn before you, so would it be fair to have the ally bullrush you, pushing you next to the enemy so you could have a full round attack on your turn? I don't think so.
I'd say it makes the most sense if allies bullrush against your full CMD because you don't have the time to think about what's happening before they slam into you. Or, on your turn, you could go on "minimal defense" as a move(?full round??) action to drop your dex/dodge bonuses until your next turn (or simply move on your turn).