I play The Pirc, and often white play d3, instead of d4. I.e.: 1.e4 d6 2.Nf3 (d3) Nf6 3.d3, or even maybe 2.Bc4 Nf6 3.d3 g6 4.Nc3 - or some other move order. The point is, white plays d3 instead of d4.

Is the d3 move from another opening, or is it just a passive/bad move from white, due to not knowing theory?

What should my reply be to this? Just continuing playing Pirc?

It might be an intentional move to take you out of your opening preparation and see if you reveal yourself as someone who's just memorized moves and doesn't really understand how to play chess.

Bad? No, it's passive though. If I were black, I would be expecting an Indian setup of some sort..

That or white is trying to lowball black out of rhythm, in which case the d3 idea is just plain bad. I'd watch my step at any rate and try to disallow 'transpositions' into other opening territory. I've seen people squirm into what could be called a Ruy Lopez with this sort of thing.

Just play by the usual Pirc rulebook and you'll be fine.

I agree with Ronaldinho 100%. It's a way to take you out of Pirc lines. Closed Sicillian is a line you'll find d3. You must do more than memorize opening moves and try to grasp the theme of the opening. Your quest should be to dominate the dark squares since you'll fianchetto the bishop.

It very well might be, yes. But, as Perseus says, it's probably just an bad idea. You give up some center pressure, and also invites black to use the d4 square as an outpost later. Of course, it takes you out of the Pirc book lines, but Pirc is more than just memorizing moves/lines, it is also a "way of playing" with a lot of ideas. I'm gonna take a look at the closed Sicilian to see whats happening there, but I think in the d3 lines I can safely still play Pirc style
As you say, those black squares are important. Do you think I must try and hold on to my blacksquared Bishop at "any cost"? You often see white try to exchange the bishops with moves like Bh6, and if my Rook is on ie e8, I could always save my bishop with Bh8. Is that a good idea, you think?

You know that's the beauty of chess nothing is set in stone - not yet. But to answer the issue of holding the bishop at all cost depends on how comfortable you feel about the g7 hole. There are lines in the Modern that black welcomes the hole - black would sometimes trade the bishop for the knight on c3! Then there's the venemous Sicillian where the fianchetto bishop seem to worth 5 points. Black would give up the rook for a knight because of the big dreams the player has for the bishop. Your pieces have to work to together - that will help you decide. Personally, I'm not afraid of the hole as I use to anymore, because it's a trade of dark square bishops. I think the real issue in that position is white holding h7 down until the file is open for the major pieces. The only way for black to defend h7 is with a knight on f8 so the Bh6 can sometimes indirectly pin a Knight to d7. Back to the point. Though d3 gives up the inititave early, it is not a bad move. It is an option- a positional one. White does not hurt its position in any way. White's goal is to dominate light squares. d5 e4 are center squares too.

Oky, I see. Thanx for the replies

Passive? It's possible. Bad opening, I don't think so. White play ?.d3 to avoid more common variations of Pirc. Black haven't problems with d3 move.
If White plays 1.Nf3 (with idea: King's Indian Attack: Nf3-d3-g3-Bg2-Nbd2-0-0-and son on) Black normally play 1...d5. In consequence, if you play PIRC defense (white d6), White can play King's Indian Attack and too haven't problems. There are many variation in Queen's Bishop (Bc8) move to g4 square (Bg4).
Here, you can see chess games in a chess viewer about this variation:
1 Nf3 d6 Reti Opening: 1...d6 variation
Lucky!