What's a pawn worth?

First, the pawn - tempo ratio is not static. True in the middlegame and endgame, it often is worth taking the pawn, even if it sets you back a move or two. However, tempo is very important in the opening, which is why gambits can be so deadly if you're unprepared. Other than some really static endgame positions, I'm having trouble thinking of a middlegame scenario where winning a pawn is worth giving your opponent 3 moves. In the opening, I would gladly give my opponent the pawn of his or her choice if I was allowed to make 4 moves as White (free Scholar's mate!) or 3 as Black! Maybe on average, a pawn is worth about 1.5-2 tempi. Bottom line, material in the endgame is very tangible stuff, so take it; however, in the opening, and even in the middlegame, material can sometimes have only arithmetic value, i.e. your Rook sitting in the corner doing nothing is technically worth 5 points, but until it impacts the game, it might as well be worth 0 and worth giving away if it means you can take away an active piece from the opponent or gain a tempo or two to press your very real attacking chances.

Also, it is important to keep in mind that sometimes it's important not to accept a free pawn in certain types of positions. The example that comes immediately to mind is when you've castled. Let's say you castled on the Kingside, and you have the option of taking a free h-pawn; sometimes it's more valuable not to take the pawn, as creating a semi-open file will provide your opponent with all sorts of attacking possibilities that he or she would not have if the file remained closed. 

Also, I am more hesitant to go for the pawn when I am underdeveloped. In a recent Guioco Piannissimo (1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. d3) I had the Black pieces. I prepared for a slow-positional game, but the opportunity to win a pawn presented itself. I did take it. However, I had to take the endgame very seriously, as I only had 1 piece developed (my Queen) where all of my opponent's pieces could quickly be mobilized. I was the better player, so I was confident I could defend properly and win with the extra material, but I knew I was going to be on my heels and backpeddling and scrambling for the next 5-10 moves. It worked out, and turns out I didn't need the extra pawn as my opponent fell for a trap when I danglged 2 free pawns in front of him that led to Mate in 2! But who knows, the whole endgame scenario could have gone very badly in an OTB situation where there is less time to calculate.

But in general, yes, always go for the material. A one-point advantage, a.k.a. being up a pawn, is all the margin of victory you need, so go for it when it's safe.