I couldn't see myself starting a combat wihen the pcs have no idea it is a combat situation. I would say that the goblins get a surprise round before the combat. However, because the events have already taken place I must side with the player. I think that the mistake came between the precombat description of the monsters and what went on in the actual combat rounds. I don't think you should make a houserule for this, else you're asking for trouble because any special priveldges the dm gets the pc's will abuse. And I'm sure there is a clause in our DM charter that clearly states we lose our membership if the pcs gain any "extra" privledges.
Seriously though
You described to your player that the goblins are in a defensive position. But is not any guard on duty in a defense position. Remember that description is not a mechanic, but just to add color to the pcs head. What your player heard was a description, however, you were thinking this is a mechanic I'm optioning. According to the rules you can not make combat decisions outside of combat, so the player was right in thinking it was flavor that you were saying.
What should have been hinted at in the before combat description is that the goblins seemed to be anticipating their attack. Now, we are still in the prebattle description mode.
Then you should have asked for initiative, but told the players that the goblins were aware of you and get a surprise round. In this surprise round, they take a full defensive stances. As they take the stances, you tell the pcs that hte goblins dig even more into their positions, readying in a defensive stance for you. I think we are getting too caught up on the defensive stance part, it is a standard action, so think about it the same as if the goblins were training their bows on the door or if they were setting up cantrips at the door.
Now, the pcs are still flatfooted by your goblins, your goblins get the defensive stance the first round and your player clearly understands why.
This way you eliminate the need for a tedious house rule (there are far more broken things in 3.5 that a house rule should be wasted on) .