It's nice to see a "native" ribbon that sticks to the Windows UI scheme in a conservative, yet aesthetic way.
I do have a few questions and opinions though:
I'm not sure that using the Ribbon with Paint was the best of ideas. Painting programs are best suited to pallettes, even if they're docked, than toolbars. Adobe's (and formerly Macromedia's) software demonstrate this. I cannot see Photoshop working with a ribbon (despite having probably more options and features than Word 2007). With the old-style version of Paint pretty much everything was one click away. Whilst with the ribbon some features are now one click away, other tools aren't: consider the Brush options. Now you need to click three times to select the brush tool, and then select a brush type. If you want to change views you need to open the View tab and then fiddle with the tools there, and then click back to the main tools tab.
Whilst I welcome moving the "File" tools over to a button that's in the tab row (rather than an "Office button"-style widget) I don't believe that programs should be painting to the non-client area at all. What alternatives are there for highlighting context sensitive tabs than painting above them in the titlebar?
Finally, you mentioned this would be available for Vista, but there was no mention of the still-supported Windows XP (and I might say Windows 2000 too). What would happen if I tried running the ribbonised paint on XP, for example?