The blades on a PC fan are not ideally suited to the application of creating lift, are they. What if he used the motor and assembly of a PC fan but cut off the blades and make some that are more fit for the purpose?
I've already been down this road of modifying a PC fan for any purpose other than cooling, and I hate to be a downer, but it's a fruitless quest. PC fans just can't generate enough useful thrust, and have a poor power-to-mass design.
It's excusable because they're not designed to provide thrust, but to slowly move a volume of air over your computer. If they were more powerful, it'd be like having a vacuum sucking all kinds of bits into your computer (which happens with dust over time anyways).
Even lower thrust applications like a hovercraft or blimp are too much for a PC fan. The blimp might work, but for the hassle for what you would get out of it, you'd be better off going for a motor initially designed for the purpose.
To give you an idea, a PC fan didn't even provide enough thrust to be used for my recorder robot, which requires extremely little thrust.
I'd guess an analogy for thrust to electonics hobbyists would be that to voltage. It doesn't matter if the fan's moving a large volume of air (current), if the thrust (voltage) is low, when the air encounters any kind of resistance (resistance) the air flow (current) will fall to zero.
It's a bummer, because there's obviously a surplus of computer fans all over the place. My advice is to get creative about using them for other cooling applications (i.e. mount one over your high-powered ICs).
If I had to go MacGyver and build my own electronic helicopter, I'd probably start with the pre-made blades on those hand-driven propellers. They're simple, but hard to describe, so I hope you understand what I mean. We sometimes get them as stocking-stuffers at Christmas. It's just a wooden dowel (about a 1/4") with a propeller about 6" across on the end, and you put the dowel between your flat hands and move them quickly in opposite directions. That generates enough lift to send the toy on a five second spin around the room. You can probably pick them up at a dollar store - they'd be cheap props, anyway, so you'd be able to justify breaking a few during the course of experimenting. I'd wager a $2 DC motor, pulley and belt drive, the prop, and a small battery pack would be able to power themselves off the ground. Definitely would work well as a drive for a blimp.
edit: Scratch the belt-drive. My parents recently brought up a load of my old Lego. I'd gear the DC motor to the propeller shaft using some of those parts. This is starting to sound like a good challenge - I just might give it a try...