Very generally speaking, phosphoric acid neutralization goes like that:

H3PO4 + 3NaOH -> Na3PO4 + H2O

50% means that you add half the amount of NaOH required. This in turn  
means that you:
1. Proceed completely with first step of neutralization, ie
H3PO4 + NaOH -> NaH2PO4 + H2O
2. Proceed 50% into the second step of neutralization:
NaH2PO4 + NaOH = Na2HPO4 + H2O

So, after 50% neutralization your solution is 50/50 H2PO4- and HPO4-2.  
This is a classic buffer solution, with pH described by so called  
Henderson-Hasselbalch equation, which in this particluar case takes form:

pH = 7.2 + log([HPO4-2]/[H2PO4-])

but we already know that [HPO4-2] = [H2PO4-] (remember, solution is  
50/50). That leaves us with

pH = 7.2

There are fine details omitted and hidden assumptions done so nitpickers  
can bash almost every phrase in this post, but no matter how the'll try,  
their final pH will be around 7 as well.

To get solution with pH around 5.5 you have to neutralize first proton and  
add very small excess of base. I would call it 35% neutralization  
(assuming 100% is when the amount of base is three times that of acid, to  
account for the phosphoric acid "triprocity").

Now, you can define 100% neutralization in terms of single proton - ie  
100% is when the reaction

H3PO4 + NaOH -> NaH2PO4 + H2O

is finished. That means full neutralization is 300%. This approach  
sometimes simplifies calculations. Still, 50% in this case is when you  
have 50/50 mixture af H3PO4 and H2PO4- - this is another buffer, with pH  
around 2.2. No matter how you try it is NOT 5.5.

Browse pH calculation lectures at

http://www.chembuddy.com/?left=pH-calculation&right=toc

if you need more. Or play with pH calculator from

http://www.chembuddy.com/?left=BATE&right=pH-calculator

to see for yourself how the pH changes for different solutions. There is a  
30 day free trial so you can do it for free.

Borek