I could think of several situations, especially in large purchase negotiations, where one party would like to know that the money is under the control of the other party before continuing with the costs of further negotiation and due diligence (e.g. $5k in a house closing).
You can sign any text with the private key you own and anyone can verify that with a public key associated with a given Bitcoin address. As long as you make the message relevant to the conversation, say "This is LethalFractal, at 2013-09-07, I claim ownership of address 1234567890... as a proof for negotiations with John Smith", the person can know that it is in fact you owning a given address, and since they can check the balance of that address, you will have proven that you have access to that money.
I don't know how technical you are or how much you care about the privacy in this matter, but you can prove that you own an amount of bitcoin without revealing which accounts you own using the Proof of Assets protocol from a paper called Provisions. You end up with a commitment to the amount of bitcoin you have and can open the commitment to prove you own a specific amount.
This is probably excessive for your needs, but it ensures that your accounts remain perfectly hidden.