Congratulations to Arianna Huffington and her investors, who cashed in big today with a sale to AOL.  The venerable on-line service will pay $315 million to buy the Huffington Post, one of the largest acquisitions in the dot-com era and the largest in the blogosphere:

AOL has agreed to purchase The Huffington Post for $315 million, the two entities announced in a joint news release Monday.

“As part of the transaction, Arianna Huffington, The Huffington Post’s co-founder and editor-in-chief, will be named president and editor-in-chief of The Huffington Post Media Group, which will include all Huffington Post and AOL content,” the statement said.

The new group will have a combined 117 million unique visitors a month in the United States and 270 million around the world, according to the release, which cited December 2010 data from the marketing research company comScore.

The editorial arrangement seems like a strange decision.  AOL has been careful to give the impression of political balance in their news and opinion offerings on line.  Putting a political activist like Arianna Huffington in charge of all AOL content will upend that balance, and might have a significant number of subscribers and readers heading for the door. The end result might be less a combination of the two readerships and more of a replacement of AOL’s community with that of the HuffPo.

Another strange decision, related to the first, is to rebrand AOL’s content with the Huffington label.  Arianna has done a great job over the last few years in building her brand in the blogosphere, but AOL has one of the oldest and most widely known brands in computing.  Perhaps renaming the content group with the Huffington label was an attempt to keep the partisan direction of HuffPo from damaging AOL’s brand, but if the content shifts under Arianna’s leadership, that won’t help at all.

It should be an interesting transition to watch, though.  None of these oddities takes away from Arianna’s success, either, in building a blogospheric empire and realizing a fortune from it.