Built in memory doesn't make sense on the Vita. The entire concept is that it's a bridge system between retail and digital distro. Selling a variety of Vita SKUs with 4, 8, etc. GB of internal memory restricts a consumer's willingness to buy more and more digital distro games as they fill up the built in memory.
Meanwhile if Sony was to go with a moderate built in number (say 4GB) and still sold cards the cards would 1. not sell fast enough to subsidize R&D, helping them bring the price down, and 2. we'd still have people bitching about whatever number Sony chooses as not being enough to store the games they want (short of Sony throwing 32GB or more in the thing, which would destroy the pricing model).
Bundling a memory card for free is the way for Sony to go. Personally I think they should have gone with dual card ports. Regardless, 4GB of memory should be in every Vita box shipped to retail. That "hidden" cost at the register is driving off consumers.