At least in the public domain, there is no evidence for this whatsoever.
Based on what we DO know, the evidence is strongly in favor of his innocense.
1. It would take $1 million worth of equipment and materials to make weapons grade anthrax. Hatfill does not have the money. Likewise, Hatfill could not have hidden the evidence if he somehow got the money and set up shop anyway.
2. If he somehow stole it, the spores would have had different size and anti-static coating characteristics than the actual powder used.
3. The attack in Florida is completely inconsistent with the other attacks if the attacker was Hatfill. However, especially since American Media had just run a very unflattering piece on Osama Bin Laden, an Islamic attacker (Atta) makes perfect sense.
4. There is a lot of publicly available information, and it all supports a radical Islamic attacker (Atta, getting it from somewhere else).
5. A very plausible, but not publicly proven, scenario is that Saddam was/is the ultimate source of the anthrax. Even the wording in the letters makes it clear that they are intended as a warning, not an attack. Used differently, even the small amount of anthrax used in the letters could have killed many more people.
6. Applying Occam's razor, the attack was exactly what it looked like: A warning. Who would feel the need to issue a warning? Saddam.
7. Applying common sense and a minimal amount of strategic thinking: There is more anthrax available to attack us with. Whenever we move against Iraq, we had better be ready for the consequences here at home.
To sum up: On balance, just based on common sense and what is publicly known, it seems very unlikely Hatfill is guilty of anything more than a (possibly) inflated resume.
You'll always have an OldFriend to kick around and if you don't there's always SubMareener who thinks the dog solved the crime.