What's frustrating about starters (or growing cells in general) is that there is conflicting information out there. Take that link that Chad posted above, it states -
"According to both White Labs and Wyeast, a White Labs Pitchable Yeast vial and a Wyeast ACTIVATOR 125 XL Smack Pack both contain an average of 100 billion cells and are enough to pitch directly into 5 US gallons (18.9 liters) of an ale wort at 1.048 SG (12°P). This is a pitching rate of 5.3 million cells per milliliter, which is close to the pitching rate many professional breweries begin with when starting a new pitch of ale yeast. This rate works well because the health and vitality of fresh laboratory cultured yeast are superior to yeast harvested from normal fermentation."
No starter needed in this instance, but if you go to Jamil's pitching calculator on the same webpage here -
Mr Malty Pitching Rate Calculator
If you put in a SG of 1.048 and volume of 5 gallons, it says you need 168 billion cells or 1.7 vials of yeast. One says 100 billion is enough, the other says you need 168 billion. Quite a difference.
I do understand starters are also meant to prove viability but isn't that why you have a smack pack? Starters for me are a pain in the "you know where", so I'm not sure if you need a starter or not when brewing a 1.048 beer.