Why use anything less than a high quality carbon????  http://www.drsfostersmith.com/product/prod_Display.cfm?siteid=6&pCatId=4250

Travis,

Those are great articles!  I'd add that you want a higher methelene blue # if evaluating different carbons (this is kind of a measure of how well it adsorbs humic material). 

I run activated carbon 24/7 and have not seen any problems with increased phosphates.  I change it out with new about every three weeks and it seems to work fine for me.

A couple articles...
http://www.aquarticles.com/articles/management/greenfield%20straughan_activated%20carbon.html

http://www.hallman.org/filter/gac.html

I would not be at all suprised if all/most carbons contain some phosphates.  Activated carbon is just coal (or charred wood or coconut shells) that is heated with (usually) steam.  At one time all activated carbon was a living thing and if there's life there's phosphates!  I'd guess not all are the same though.  Also I'd guess that most aquarium product vendors are smart enough to sell one that doesn't leach.

(btw, I met with the local rep for Norit* about two weeks ago... he told me he could get me a good deal on activated carbon... but I'd need to buy a dump truck full )
*Norit is by far the largest manufacturer of carbons in the country.  Most likely the carbon you're using came from Norit.  Oh, and he wasn't visiting me to sell activated carbon (it was something else) I just saw Norit on his product sheet.

I've got a bucket of Kent's Reef Carbon and it says "Will not leech phosphates" which indicates to me that it does have a phosphate base. As to letting the phosphates escape into the water....I personally am not  too worried about it. If you are only using carbon a couple of days a month then it would have to be major leeching of PO4 to stimulate algae growth. Also, it it was leeching then you would get good growth the about week after using the carbon, after which growth would decrease as the posphate is utilized (assuming minimal leeching). If you run carbon constantly and are worried about it, reduce the usage and see what happens.