The Boston Museum of Fine Art, which has put nearly 340,000 pieces from its collection online, has wallpapers for your mobile phone available for purchase on its website. (I really like this one, Odilon Redon's Large Green Vase with Mixed Flowers.)
Is it worth the $1.99 (plus text message fees) that gets billed to your cell phone? I guess it depends how much value you put on your phone's appearance. I'm happy using snapshots I've taken as my wallpaper, but I might change my mind.