Though the question is off-base for the ux forum, one of my clients is a web service for doing contracts that include e-signatures. The short answer is that the there are no legal issues with signing on a tablet. A short history of e-signing and its roots from the age of the telegraph is posted on my client's blog:
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That said, there are some UX issues to consider. The first is that people need to be made aware that they are entering a legally binding contract by signing. Click-through is legally binding, but feels a bit too easy to be serious or real, and the UI needs to address that perception.
Secondly, people need to feel secure in their signature, that the signature can be validated in some way. Communicating this in a digital product without resorting to technical terms is hard to do, but important for people to trust an electronic signature on a tablet or other device.