Hate to be a beginner, but i just bought LR3 but worry about what i do with DNG files to share with others via e-mail or picassa. Is there a way to convert back to jpeg for sharing? i tried uploading dng to picassa for the heck of it, but it wouldn't work. Can't start converting everything if i'm the only one who can enjoy the pictures!
DNG files contain the raw image data. They are too large for sharing by e-mail or for posting on websites. Use the Export feature in Lightroom to export appropriately sized JPEG images and post those images on your website or send them by e-mail.
DNG is in name and function a digital equivalent to film negatives.
You probably wouldn't give your friends a dozen copies of a single negative strip, you'd make a dozen prints. It's easier for them to handle and view.
Metaphorically this is the same as DNG versus JPG. One is the original data from the camera, one is exported to share.