NL50: AA limped UTG, fold to river bet?

[AlexB182]

Though I would normally raise with AA here, I sometimes go for the limp/reraise to mix up my play, especially at rather aggressive tables. Obviously, I didn't work out here as planned.
Like to hear some comments on the flop play, I was going for the c/r hoping to get the pot HU. At the turn, I hope BB is just taking a stab and I can take the pot down here without much resistance. If called, I think I'm pretty much done with the hand.
BB is 24/4.8/o.53 over only 41 hands, nothing unusual so far.
MP2 is 18/3.5/0.85 over 115 hands, pretty weak tight.
All comments welcome, though I think we don't have to discuss the PF play.

Full Tilt No-Limit Hold'em, $0.50 BB (9 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: 2+2 Forums)

Button ($37.40)
SB ($74.10)
BB ($18.95)
Hero ($40.75)
UTG+1 ($41.90)
MP1 ($16.60)
MP2 ($27.55)
MP3 ($41.85)
CO ($31.05)

Preflop: Hero is UTG with A[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/diamond.gif[/img].
Hero calls $0.50, <font color="#666666">2 folds</font>, MP2 calls $0.50, <font color="#666666">4 folds</font>, BB checks.

Flop: ($1.75) 9[img]/images/graemlins/spade.gif[/img], 3[img]/images/graemlins/club.gif[/img], K[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(3 players)</font>
BB checks, Hero checks, MP2 checks.

Turn: ($1.75) 5[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">BB bets $1.5</font>, <font color="#CC3333">Hero raises to $4</font>, MP2 calls $4, BB calls $2.50.

River: ($13.75) J[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">BB bets $3.5</font>, Hero ?