Checking the river

Full Tilt Poker Game #_: $10 + $1 Sit & Go (_), Table 1 - 15/30 - No Limit Hold'em - 1:58:52 ET - 2007/12/17
Seat 1: (3,435)
Seat 2: (975)
Seat 3: hero (1,620)
Seat 4: villain (1,470)
Seat 5: (1,500)
Seat 4 posts the small blind of 15
Seat 5 posts the big blind of 30
The button is in seat #3
*** HOLE CARDS ***
Dealt to hero [5d Kd]
Seat 1 folds
Seat 2 folds
hero calls 30

In position with a suited king and no limpers or raisers. I suppose I could have raised here, but it's early yet.

villain calls 15
Seat 5 checks
*** FLOP *** [9d 2d 4d]
villain bets 90
Seat 5 folds

Nice. The 2nd-to-nut flush and the opposition is betting into me. Let's trap.

hero calls 90
*** TURN *** [9d 2d 4d] [Tc]
villain bets 270
hero calls 270
*** RIVER *** [9d 2d 4d Tc] [Jd]
villain checks

There are two possibilities. Either he has the ace of diamonds and is getting ready to check-raise, or we have him beat. But look at it from his perspective: There are four diamonds on the board and no pairs. If he has either a small diamond or no diamond at all, is he going to call a bet here? What's the upside if I make a bet here? Nobody is likely to just call. Either they're going to raise with the ace of diamonds or fold. If the ace of diamonds is trying to check-raise, then I save chips by not falling into the trap, and if nobody else is going to call, I gain nothing by betting. What do I do if he raises? Fold, having probably lost another 600 chips or so? At the very least, checking it down let's me show that I was trapping with a good hand when I called on the flop and turn and will hopefully improve my table image.

hero checks
*** SHOW DOWN ***
villain shows [9c Ah] a pair of Nines
hero shows [5d Kd] a flush, King high
hero wins the pot (810) with a flush, King high
*** SUMMARY ***
Total pot 810 | Rake 0
Board: [9d 2d 4d Tc Jd]
Seat 1: didn't bet (folded)
Seat 2: didn't bet (folded)
Seat 3: hero (button) showed [5d Kd] and won (810) with a flush, King high
Seat 4: villain (small blind) showed [9c Ah] and lost with a pair of Nines
Seat 5: (big blind) folded on the Flop