There are plenty of companies that get acquired before having a solid business proposition. Jaiku (along the lines of Twitter) is a recent example, but there are many others. A company can be acquired for a variety of reasons. Maybe the team is a good fit for the parent (talent acquisition). Maybe the product is hard to monetize on its own but it's a good feature that will increase the value of something else (could have been Xobni).
Perhaps it's a loss leader, a service that you plan to give away in order to attract customers to your main business (a contrived example, Godaddy could buy some game that somehow encouraged you to buy domain names).
"There are plenty of companies that get acquired before having a solid business proposition."
That doesn't make it a good idea. The answer to "Who is your target market?" should not be "Uhhh... Google?" There are many more companies that built a "cool" product that was useful and used, couldn't monetize it, and died because of lack of capital. Somehow I doubt YC has ever invested in a company that flat out had no exit strategy outside of being acquired by a big company. You've got to have something because if none of the big fish bite you're screwed otherwise.
Perhaps it's a loss leader, a service that you plan to give away in order to attract customers to your main business (a contrived example, Godaddy could buy some game that somehow encouraged you to buy domain names).