I think nominal voltage for li-ion is 3.7 and li-poly is 3.6, I'm not sure but anyway it's close.
A fully charged 8 cell tx pack reaches 11.3 volts whereas a fully charged 3S lithium pack reaches 12.6 volts. 1.3 volts may not seem like very much but I'd rather not have to buy another tx, I like mine very much.