Autography is probably the best well known, and I have to say it is the slickest. The system is based on iPad and Android apps which are tied into Autography's ebookstore. They're looking for partners, but right now autographs can only be integrated are integrated into (DRM-free) ebooks they sell.
I think Autography is the one most likely to hit the market first.
Inscribe Media just debuted thier system today and it's going to launch later this year. This system was developed by an independent publisher who wanted to be able to sign ebooks. Inscribe are actively looking for partners who want to integrate Inscribe's system into their ebookstores. Inscribe has a system less flashy than Autography's but that might be a plus.
iDoLVine just popped up on my radar last week, and it should launch later this year. This company is going for more than just the autograph. They want to digitize the entire con experience, which is why they've built their system around webcams, remote signing, and more. Thsi is the most ambitious sytem but it is also jkept having technical issues. Sometimes more is not better.
The lead photo shows Neil Gaman remote signing for someone with the iDoLVine system.
I'm planning to do separate posts for each. But I have to say that from the user's perspective, Autography is the best. From an ebookstore, Inscribe might be better,