Why are recumbents so slow uphill? I've ridden a recumbent a few times and noticed that going uphill is very, very slow. Given that recumbents are supposed to be more efficient than diamond frame bicycles, why are they so darn slow uphill?

Related: does "efficiency" mean something different when applied to recumbents?
Bike geometry and bio-mechanical topics have been discussed by others already, but I wonder if a large part of what you're observing is a relative difference between speeds on the two types of terrain?

Think of it this way:

You ride a "regular" bike on the flat. Your ultimate top speed is essentially governed by aerodynamics. You reach a speed where the force of the air you're pushing against equals the force you put in.

When you ride a recumbent under the same conditions, it's more aerodynamically efficient, so your top speed is higher. 

Makes sense.

Going uphill it's a little different.

A larger part of your input energy goes in to overcoming gravity. Which is why your top speed will be lower.

Riding a recumbent uphill, you're still overcoming exactly the same amount of gravity, so aside from other mechanical advantages a recumbent may or may not give you in this situation, your top speed will be roughly similar to riding any other type of bike. The aerodynamics play a much smaller part in the equation..

Summary : the relative difference in the two top speeds is going to be greater  (and more noticeable) on a more aerodynamically efficient machine.

Caveat: I'm neither a scienctist, nor a recumbent rider :P