Have a look at
www.seven.com
They have a web-based service. You give it the details of your email service, and they poll your account as often as you specify, and then send it to your device via GPRS. I got the trial version working quite easily with my Imate Jam, using a Moscow mobile provider. It looks like it would work with Yahoo if they allow outside clients to address their POP servers. I know google allows this.
The full version costs about $50, I recall. I didn't buy it, not because the product isn't good, but I realised that I preferred to "pull" my email, than to have it pushed to my device.