It sounds like going the free route was a mistake. Bootstrapped startups have to charge something to somebody, and you-pay-me-for-my-product is a time tested way of doing it.
As for the rest, you need to tamp down your costs. $30/day is extremely high for a bootstrapped, no-revenue startup. Find a way to cut this down. Based on your comments about GAE and SQS, see about moving to a standalone VPS (Slicehost or whatever) and running your own ActiveMQ server, probably on the same machine. It's hard for me to imagine a web app w/ less than 10k users needing more than a single big VPS.
Also, we ran into the surprise of discovering our potential userbase: the 12-18 crowd stormed in like bulls in a china shop posting sexually explicit pictures, profanity, etc. which created a whole new slew of headaches. It's a valuable/lucrative audience, but you need a fair amount of policework to prevent things from getting out of control...
One of the fellows at SmugMug has blogged about the "free" question, and pointed out this particular problem with "free". When it's free, you capture a part of the market that most wants anonymity and no recourse for their bad, possibly illegal, behavior. 12-18 year olds exchanging sexually explicit pictures via your service is a problem of epic proportions. Stopping it could very easily become your full-time job (and the full-time job of a dozen more people).
I know you've said you've already halted free distribution. I just wanted to point out that this one is a well-known problem with free, and if you don't have good, automated, methods for dealing with it, it can be a huge resource drain.
Market validation is once you have the data to say "customers in demographic X will pay Y dollars for this app, I will make a margin of Z percent, and there Q customers willing to pay."
Unless you can survive on your free version (which it sounds like you can't), you haven't validated the market yet. Find the price point where you are profitable (and people will pay!)
http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewSoftwa...
Do contract work to float yourself. And charge for your app. Integrate with Twitter, Facebook chat, etc. OK, I'm done my rant, sorry.
If you've built a successful iPhone application, you're a marketable contracting operation. Stop talking about getting funding.
Which naturally raises the question: "Doesn't the iPhone already have a perfectly usable chat application built in?"
So you need to change something.
It sounds like you couldn't afford to release a free version, with all the costs involved. So don't. Stop releasing the free version. Think of a price you would be comfortable charging for it.
Now double that price. Chances are you undervalued what people might be willing to pay for your application. You have a growing user base, but most unfortunately if it grows too much you'll just be out of business that much sooner.
Sure, it wouldn't hurt to keep talking about acquisition. The beauty of that is in most cases these talks take forever, so you have plenty of time, and can always back out or tell them you'd like a few more months. The downside to this is they might just wait until you run out of money, and then whatever small amount they were offering you will become surprisingly smaller. When they have a meeting about whether to acquire your product or not one of their strategies is going to be 'let's wait and see what happens in the next couple months'. It's only to their advantage.
And at least give us the link so we can let you know if we would pay for it! Your company is failing, this is not the time to be discreet.