I have a piece of aluminum with an electrical heating element in it and was wondering if I could use a light dimmer switch to control the current to the element to regulate my heat output or do I have to get a rheostat and why.

No you can not use the dimmer switch. It is "Listed" for light circuits only. You cannot use it to control fan speed.

Electrically speeking, that type of switch could not handle the current draw needed by a heater.

A lighting dimmer switch is not a rheostat.

Rule #1 Never listen to people that do not know what they are talking about! Lol

Lighting dimmers are designed for resistive loads. Heating elements are resistive loads! The nickel chromium wire in the heating element is the same type of load as the tungsten element in the light bulb, A resistive load. As long as the dimmer is rated above the wattage of the heating element it will be fine. They make dimmer switches rated for 5amps @ 120VAC for $10 bucks on E bay. A dimmer rated at 5amps continuous will easily handle a heating element up to 600 Watts. I'm pretty sure you did not mean a 1500watt room heater, that would be a stretch unless you wanted to spend big bucks on a heavy duty unit made for stage lighting or something. Motors on the other hand are inductive loads and they require a fan controller. Never use a dimmer on a motor with small shaded pole motors being the exception to the rule. Normally inductive motor speed is determined by the frequency of the alternating current not the input voltage. Brushed motors will also work on a triac based dimmer but that's where it gets confusing. It's better to stick with dimmers for resistive loads and fan controllers for motors. You can figure out the wattage of your heating element by measuring the resistance with an OHM meter and the operating voltage it was made to run on. Say for example it reads 48 ohms resistance and it is designed to run on 120 VAC it would be a 300 Watt heating element. Such an element would easily be handled by a 5 Amp lighting dimmer. I hope this actually helps you out more then someone just saying it won't work or pointing out a dimmer is not a rheostat like you already knew. You could actually use a rheostat too but it would have to be a seriously beefy rheostat and they cost a fortune. A dimmer and a rheostat do function nearly identical but the dimmer uses a solid state circuit to vary the voltage where a rheostat physically adds more resistance and the windings must be able to endure the same current as the intended load. Motor controllers function by pulsing the power cycling on and off rapidly the more time inserted between the pulses the slower the motor runs. They also have a trim adjustment you can set to ensure it never adjusts slower then the motor can actually start spinning as that can cause overheating issues, yet another reason why you should never use a dimmer for motors as dimmers do not have that feature. Another choice would be a variac(AKA auto transformer) They are nice to have around but again they are costly. They isolate your project from the mains for an added layer of safety. One last thing, Dimmers do not work with the new CFL bulbs or any other fluorescent lighting. Wireless remote on/off switches do not work with them either, they just go from on to dim when switched off. I found that out trial by error... lol Take care and enjoy experimenting...

... so are lighting dimmers also "listed and labeled" for control of devices other than lighting fixtures? NEC 110.3(B) may be at odds with this...
Of course if this for personal experimentation, you may use anything at your own risk...

A lighting dimmer is not a rheostat, does not operate the same as a rheostat, and is listed by Underwriters Laboratories as a Lighting dimmer.

Anyone using a light dimmer as a rheostat to control electric heat will do so at their own risk.

HowStuffWorks "How Dimmer Switches Work"

Here is your answer. You can take the risk of starting a fire of BUY a "Dial-A-Rehostat" for about the same money you would spend on a little dimmer. I am buying one so I can control my electric smoker. Here is a link to the product... http://www.countrystoveandpatio.com/shop/dial-a-temp-rheostat/?add-to-cart=368