I have had several questions about InfoPath 2010 and support for Digital Signatures. The answer is yes, InfoPath 2010 supports them and there is even more improved capability. Here are some good references.
- This provides good general information about InfoPath Digital Signatures - http://office.microsoft.com/en-us/infopath-help/digital-signatures-in-infopath-2010-HA010381868.aspx
- This provides information describing in more detail the improvements with SharePoint and InfoPath 2010 - http://blogs.msdn.com/b/infopath/archive/2010/02/18/digital-signature-support-in-infopath-2010.aspx
- This is a good article about the event model around digital signatures and how you create customizations around those event - http://msdn.microsoft.com/en-us/library/aa948780.aspx