RAM is insanely, insanely cheap. To make any money, computer makers are going to have to configure machines with lots of RAM and disk and try to make margins on that upgrade. Dell already charges about $150-$200 more than Newegg for 4GB of RAM, for example.
Why will this drive 64-bit? Frankly, because people want to use what they paid for. Someone gets 4GB of RAM in their machine and they run out of memory running Firefox with a bunch of Flash apps. Or they just see a little RAM meter that says they only have 3GB of user-mode RAM in the machine (in the case of using the /3GB switch). Ultimately people are going to ask how they can use the whole 4GB, and the answer is to upgrade to 64-bit. For most users, this will probably be fine.... as long as they don't try using shell extensions (e.g. right clicking for 32-bit Winzip commands is an example of a shell extension that's lost).