The Giant Book of How To Do Your Individual Taxes, actually known as Publication 17, has a little bit of guidance in the Signatures section:
You must sign and date your return.
Unable to sign. If the taxpayer is mentally competent but physically unable to sign the return or POA, a valid “signature” is defined under state law. It can be anything that clearly indicates the taxpayer's intent to sign. For example, the taxpayer's “X” with the signatures of two witnesses might be considered a valid signature under a state's law.
That gives us something to go on, in that in some circumstances at least what counts as a signature may depend on your state. I'm not sure why you're asking this question, but if you're asking because signing after printing would be physically challenging for some reason, this indicates that you may want to check your state law (or tell us your state) for some more specific direction.
In general, I would expect that in practice the person processing the return at the IRS just glances quickly to see that there's something reasonable there, and doesn't do much with it other than probably mark in their data processing system that it was signed, and I assume that they either take digital scans of submitted forms and/or store the originals in case there's a question on it. I wouldn't expect it to matter legally whether the signature is with a ballpoint pen, fountain pen, printed, or even in crayon, just that it was a signature the taxpayer produced via his or her own direction with the intent of signing and attesting that your return is complete and accurate to the best of one's knowledge. As long as there's not some sort of fraud involved, it shouldn't be a problem.
The other way to have a digital signature, of course, is to skip the paper entirely and file electronically.