There is much debate on whether players are willing to shell out $20-$45 per book, for a completely different system. If the new rules are better then 3.5 and the price is competitive the conversion rate will probably be pretty good. However, if Wizards wants players to pay $35 for 3 different core books (similar to DMG/PHB) to even begin playing, those who purchase the new material will generally be kids who live in their mothers basement and die hard fans.
My thought on the whole subject is that dnd 4th edition will not be a big improvement over 3.5 because it’s going to be a completely different game. In order to make 3.5 obsolete and regain “profitability” without third parties coming in, dnd 4th edition is going to have to completely rewrite the ways rules, timing, combat, characters, dm made characters, encounters, and loot are created while still keeping the classic spells useful (magic missile), and changing or removing the broken ones (polymorph).
Another thing WotC is going to have to change to make Dungeons and Dragons 4th edition desired is make character progression simpler. Right now there’s about 27 base starting classes anyone can choose from and about 145 different prestige classes. It literally takes hours for a new player to understand the rules enough to even begin playing the game. From learning the classes to choose from, to choosing the feats, to rolling up the stats, to researching the equipment needed, to understanding BAB, size modifiers, and stat modifiers, the game is incredibly complex before you can even begin to “role-play.” In the end the rules will have to be made simpler or the current DND fans and those coming aboard for the first time won’t want to go out and buy the new product. If it’s too hard, neither audience will convert, but if Wizards can create a game that’s easy to learn and fix have gaming sessions that don’t screech to a halt when someone wants to roll a grapple check, I’m sure the game will be a hit.