Tell me what computer will fit on a music stand of a music keyboard and store every piece of sheet music / chord charts / lyrics / set lists a muso may have and not look stupid. OnSong + iPad becomes the perfect songbook for live stage use.
It's a good point, and there's an opening there for something like that. However you won't see too many professional musicians using them for displaying their scores - the shiny screen makes the risk of glare too great. Now, an iPad-alike and sized device with an e-ink screen would be the business...
/me runs out to patent that idea.
Interestingly, on a recent holiday in Portugal I was impressed in one restaurant to have the winelist handed to me on an iPad. Fantastic idea! just swipe through them to browse, or sort by region, grape, price etc., read about the winery and other details you don't get from the traditional paper ones...