You can buy USB keyboards with just the numeric keypad. That sounds like what you want.

(I assume that some models also include the ()-=+ keys for completeness)

Does the iPhone have an host USB port? I don't think it does.

There are bluetooth numberpads, though, those should work. Something like this: http://store.apple.com/us/product/H0321VC/A (just found it, never used)

From what I gather, the iPhone does have a host USB port (assuming you have the Camera Connection Kit to provide the connector), but it is not enabled in iOS.

iOS on the iPad is said to have USB keyboard support. I don't have the necessary hardware to back that up though.

You can plug a variety of USB gear to the "camera connection kit" on iPad and it just works, from USB headsets (audio + mic) to keyboards.

Is that something you would pay for? Physical interfaces and keys are something of an obsession of mine (I don't use any keyboards that don't have Cherry MX switches, for example).

Yep - I'd definitely pay for a stylish hardware icalc keypad over a whole calculator. Im studying mech engineering. So it needs to be more than just a numeric keyboard - a scientific calc layout with brackets, exponents, logs and trig functions at a minimum. I guess the ultimate would be e-ink keys that you could relabel, but it wouldn't be too bad to use the screen for rarely used functions and just have a sensible set nicely laid out in hardware.

It sounds like someone needs to introduce you to computer algebra systems. I recommend Mathematica, Maple, or Matlib.

Slightly different issue. Entering equations quickly and accurately is very hard with a virtual keyboard. I'd like to use my phone rather than a calculator in situations when i dont have a full pc handy. I've used Maple extensively and it's a great tool. But I'm not looking for a pc replacement, I'm looking for a calculator replacement. The app side could certainly work like Maple or Mathmatica.

So you are willing to carry around a full mechanical keyboard to attach to your phone and type complex equations, but you can't carry around a netbook or something for that purpose?

I have to say, I'm a little baffled. Phones are for phoning. Computers are for computing.

Smart phones are computers that are capable of phoning too. Netbooks and laptops aren't ideal for entering a lot of numbers either because they don't have a numeric keypad. Calculator keyboards are laid out better for working with equations.

Let me put it another way - do you think calculators are completely redundant?