Capital One is now offering one of the richest mileage bonuses ever — in addition to the 10,000 mile signup bonus you normally receive for signing up for their Venture Card (and spending $1,000 within 3 months), they will also match up to 100,000 miles you have in another frequent flyer program.

FF programs that are eligible for this matching reward include:

• Alaska Airlines
• American Airlines
• Continental Airlines
• Delta
• Frontier Airlines
• Hawaiian Airlines
• JetBlue
• Spirit Airlines
• United Airlines
• US Airways
• Virgin America
• AirTran Airways (get 1,000 Venture miles per A+ Rewards credit)
• Southwest Airlines (get 1,200 Venture miles per Rapid Rewards credit)

This is a limited time offer, ending May 13, 2011. The bonus will likely expire sooner, as Capital One is also capping the number of matching Venture miles it will issue as a part of this promotion at 1 billion.

It should be noted that Capital One Venture Miles are really just credit towards future travel, at a value of 1 mile per cent. So, 110,000 Venture Miles is equivalent to a $1100 in travel credit.

After getting your approval for the card, you’ll need to submit proof of your current mileage account balance and then spend $1,000 on the card within 90 days. Existing customers who upgrade to the Venture card aren’t eligible for the 10,000 mile bonus, but can still get the matching miles.

There is one important ambiguity in the terms and conditions. The FAQ suggests that you must show that you earned your 100,000 miles from an airline co-branded credit card, but it seems likely that you only need to show that you have 100,000 miles in your qualifying FF account.

Is the card itself any good?
The Venture card itself does have its merits, as it carries no annual fee, but essentially boils down to a credit card that will give you a 2% rebate, with the restriction that your rebate can only be spent on travel. As with all Cap One cards, it does not charge any foreign currency transaction fees.

For more info or to apply, see www.capitaloneventure.com.

Edit 4/5/2011: This promotion has now expired, as all 1 billion miles have already been reserved. However, Chase and BA have just begun their own 100,000 mile promo.