RIght from the US air website. The link will give you the whole list of bennies, but I included the part about the credit card earned miles converting to Preferred miles. You do not lose any redeamable miles.

http://www.usairways.com/dividendmil...reditcards.htm

Apply today for the US Airways Visa Signature Card
Apply today for US Airways Premier World MasterCard

(1) Double miles do not apply to the purchase of US Airways Vacation
(2) Spend $25,000 or more in a calendar year and 10,000 miles you have already earned will count towards Preferred/Elite status.
(3) Valid for the first year you have a card. After the first year you will earn 2 miles per $1 spent on US Airways and US Airways Vacations purchases and 1 mile per $1 spent on other purchases
(4) Visit usairwaysvisa.com for full terms and conditions
(5) Visit lotsmoremiles.com for full terms and conditions

Bank of America also offers US Airways Visa Classic, Gold and Small Business cards. Learn More.

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All Rights Reserved
Still can't answer your question on tax payments.
I have also heard that if you have more than one US credit card, you can get the 10K on each card with a 25K total usage.
Eileen