Often backwards pawns tend to be created somewhere after the opening - there are only one or two theoretical positions where a side willingly allows this.
Sicillians are the example that springs most readily to mind - black often plays d6 and e5 in the sicillian, giving himself a weak d6 pawn, and also allowing a white piece to land on the d5 square.
In compensation, he tends to get more activity with pieces - frequently it is sufficient for him to play d5 and free up his pawn. Also, activity on the queenside, and sometimes playing f5 gives some play up the f file. A good black player will not allow mass exchanges of pieces, otherwise the weakness is exploitable.
Secondly, some lines of the french (the tarrasch springs instantly to mind) give black a backwards pawn on e6. This is actually not as bad as it seems as it is relatively easy to protect, and black usually gets good counterplay up the f file.
So the lesson to be learned is that positional weaknesses in openings are a form of bait - you know white will have a go at exploiting them during the middlegame but as black you want to be creating tactical opportunities wherever possible to keep yourself in the game.