Wellp.
At my store (which I've owned since April 1, 2007) there were roughly 20-25 people I could count on to buy 3.5 books when they came out. There were about 18 people who had weekly games at the store every week in about 4 groups.
Since September of 2008, when I started hosting 4E RPGA games at my store, I've seen those numbers grow exponentially. I now have 20-30 people at my store *every single Sunday* for two slots of D&D and every Thursday for one slot. I got parents bringing kids in, husbands bringing wives, boys bringing girlfriends; I got the 40-somethings undergoing their second coming, and LARGE numbers of new players who find 4E's simplicity to get into a huge boon over deciphering 3.5E books and rules.
So...yeah, I'd say, from my perspective, that D&D 4E is awesome for bringing in new players.
Just by way of comparison vis a vis copies sold:
Magic Item Compendium: 15
Monster Manual V: 18
Rules Compendium: 16
Adventurer's Vault: 37
Monster Manual: 45
PHB: 96
I know that the comparison is indirect, due to the "enthusiasm" factor and the "end of the production cycle" factor, but it's still been a huge boon to business and to new player purchases. Frankly, though, people are clamoring for new 4E content and seem to really enjoy the game.