Just in terms of volume of materials used, it's clear, to me at least, that if every household had an 8x10 digital photo frame, vrs every household owning a dedicated photo printer along with attendant ink cartridges, boxes of inkjet paper...the waste stream is continuous with a photo printer.
The digital photo frame is clearly the way photo display is moving. Excitement over the IPad in Fine Art photography circles is over it's use as a portfolio. Recent industry pundit articles (Thomas Hogan) discussing "whither cameras" argue that the next development required to restart sales (the market is reaching saturation, and digicams are about to be made obsolete with improved camera phones) will be the communication (digital) between the photo making device and the storage and display devices.
Finally, referring to professionally printed photos vrs digital photo frames is a false argument...Fine art printing, with color balancing, tonal corrections, localized adjustments, and color managed output, is hardly typical of what you get when you send your snapshots off to Costco or wherever...Better pictures result from becoming a better photographer, not whether output is displayed on paper or screen.