This is like the ridiculously awesome deal they had in July, except better. US Airways is offering a 100% bonus on all miles purchased or transferred through the end of the year (maximum of 50,000 miles per account).
As Gary points out, that means you could fly between the US to Europe in business class for around $1,000. Just buy 40,000 miles and US Airways gives you 40,000 more, enough for such a ticket.
I know a lot of people that were angry at themselves for not taking advantage of the offer last time, so don’t let it pass if you think you might have a use for the miles….