According to a developer from Acclaim, the still unannounced PSP 2 will be ditching the UMD format. From his Twitter and Facebook accounts:
I hear Sony FINALLY has the PSP 2. And thank goodness, they’ve removed the stupid battery-sucking UMD disc drive. I’m excited!
No UMD wouldn’t just mean better battery life. Its absence could allow for thinner form factors as well. Thinner sliding form factors, perhaps? [Kotaku]