I think Nimbuzz does yahoo as one of its clients, it does all IM's on the market i think... http://www.nimbuzz.com/en/android/download
There are a few in the Market. I'd browse there and see which ones show available then come search here to see what people say about that application. Google luck, I haven't used any Y! yet.
I tried Yahoo Droid today. It's YIM only but seems to work well. Easy to use. Free in the marketplace.
I wrote an e-mail a while back asking yahoo to make an official yahoo IM app for Android and they emailed me back saying they do not currently have an app for Android but nay consider one in the future. I doubt it though since Yahoo and Google are competitors
