yep, that's cool. lots of people do that. you'll still need to cool the wort a bit before you mix, so you don't crack your carboy, and because you can't get your top off water cold enough to cool almost boiling wort to pitching temps. but, if you whirlpool and let it sit for a few minutes while you set up your racking cane and sanitize your car boy it should cool down to 150 or so. if you're gonna try it do a little math ahead of time so you'll know what to expect:
wV(wT) + aV(aT) / (wV+aV)
w=wort
a=top off water
V=volume
T=temperature
example:
2.25 gallons of wort @ 150F added to 2.75 gallons of water @ 40F
2.25(150) + 2.75(40) / 2.25+2.75
337.5 + 90 / 5
427.5 / 5 = 85.5F
you *could* pitch at 85F, as long as you continued to cool, but i wouldn't recommend it. but now maybe you could just use a cold water bath in the sink instead of the ice, and cool your primary in the fridge (40F).
sorry for being so long winded, hope i make sense.