does anyone knoiw if I can change my mailing address on line for my tax refund?

Answer 1

If you are in Canada, CRA's website explains how to change your mailing address:

As per CRA:

How to change your address
If you move, notify us of your new address as soon as possible.

Reminder
If you receive a Canada Pension Plan or Old Age Security benefit, you should also change your address with Human Resources and Social Development Canada.

By Internet
You can change or view the address and telephone number information we have on file for you, using our My Account service.

By mail or fax
Send your request by mail or by fax to your tax centre. Be sure to include your:

signature
social insurance number
new address
the effective date
By phone
Call our Individual Income Tax Enquiries telephone service at 1-800-959-8281.

Why is it important?
When you tell us your new address in advance:

you can avoid any disruption to benefit payments you may be receiving, such as the GST/HST credit, the Universal Child Care Benefit, the Canada Child Tax Benefit and benefits under any related provincial and territorial programs.
next year we can mail your return package to the correct address.
Notes
Please note that we will not forward your new information to other government departments, except Elections Canada, but only if you gave us your authorization by answering "Yes" to that specific question on page one of your return. If you want other departments or organizations informed, you must contact them directly.

If you use our direct deposit service and you have a new account at a financial institution, please also inform us of your new account.

Hope this helps you.

http://www.cra-arc.gc.ca/tax/individuals/topics/income-tax/changeaddress-e.html

Answer 2

Just got an email from the IRS tax tip section today on this. See http://www.irs.gov/newsroom/article/0,,id=107704,00.html

It's not totally clear whether you can change the address BEFORE your check has been returned to them as undeliverable, but you could give it a try.