They would be crazy to sell now
As a developer who has started using MongoDB recently. I can't tell you how great a product they have. Its a perfect mix of NoSQL architecture with many of the features of a regular SQL DB. I have treid several other NoSQL DBs and this is by far the easiest to work with. Not only that it scales so well and is so fast.
Sure Oracle is better for situations were multi record transactions are essential but a lot of the time they aren't needed, particularly on the web. They also don't scale well.
10Gen also seems to be doing everything right to build a viable business around the product. If they were to sell now they ought to make sure they get a really good price. This company really has a chance to be the next Oracle.
Selling now would be like taking the first billion offered for Facebook. They should bide there time and ride it all the way to a later IPO.