Does anyone have any ideas as to what an appropriate tip is for someone who helps you with a wheel chair at an airport?

Depends on the economy - what country are you travelling from/to??

In the US or Western Europe.

In Western Europe I imagine a $2 - $5 equivalent would be reasonable, but would also be rather unexpected. It also depends on how long you receive the service and how difficult the journey is - some airports are trickier than others.

The US I guess is higher!

In the UK a genuine "thank-you" would be plenty.

The rest of Western Europe has less of a tip culture than the UK so a merci, danke, gracias, grazie etc should be equally well received.

You tip, they'll think 'typical American chucking their money about' but happily take it. You say "thank you", they'll think 'nice person' and go about the rest of their day feeling they're making a small difference in the world.

Enjoy your trip.

A guy in one of my classes works at ANC "doing wheelchairs." He said that he appreciates a dollar or two but that he is perfectly content with a nice smile and a sincere thank you. He mentioned that one time a lady insisted he take $5 and he thought that was too much.

Thank you all. I should have added that I have found some whell chair attendants appearing rather insulted when given a tip of $5 in the US. I was wondering if I was off the mark or if they were trying to "rip me off". From your comments it appears that it is the later.

Definitely no need to tip for that in Europe!!! (can't comment about the US)
Being nice to them and showing gratitude (non-economical gratitude) will be very appreciated though.
If you thought they were nice and helpful, just tell them!!

I can't imagine tipping anywhere for that, it's their job, and they're paid a regular wage, not a tipped wage. If you really wanted to do something, I'd say maybe $2 or $3 in the US. But I'm really against the whole tipping culture in the US. I mean I tip generously for food, etc, but I like the way it is done in Europe much more. I have a hard time tipping people who don't depend on tips as a part of their income. I certainly can't imagine tipping for it in Europe.

I totally agree! I personally hate the whole tipping system.
Whenever there is a question here about tipping, well I can't really do much about the way it works in N America (and it's not my business either...), but I can ask for example Americans coming to Europe, please please don't bring this exagerated tipping thing here!!
I don't want to live in a country/on a continent where I am expected/forced to pay money to every single person I meet; to the waitress to clear my table, to the maid to clean my room, to the shopkeeper to help me with my purchase, to the bartender to get me a beer i.e. pay people who are just doing the job they are paid for.
I want to have the liberty myself to decide when to adjust the bill if and when the service was very good, without having an obligation to do it.

What happened to the good old compliments??
Just telling people they were nice, helpful, friendly. Sending letter of compliments, telling the waiter the food was delicious and the service excellent, complimenting the shopkeeper and teeling him/her you would happily recommend the shop to others etc.
That will be so much more appreciated than opening the wallet all the time.