With a lot of work, a lot of overhead and little gain, you could potentially use it through a Wrapper Plugin with Unity PRO yes. But otherwise, unless you are a bussiness with real funding and license the source code, I don't think its worth to even try it because the overhead will be massive and some stuff will potentially not work at all (the whole animate part for example will be more than troublesome, different nonfree havoc libs won't work at all)
Out of curiosity, why would you want to use Havok physics in Unity? I could understand wanting functionality similar to the Havok animation state system but Unity is still a budget engine compared to others with Havok or similar physics (Source, Unreal3). Most companies that have the budget for Havok have the budget for one of these engines. Having said that, for an affordable engine the physics in Unity are already incredible. No disrespect intended, just asking questions to learn more things.
And for clarification and peace of mind to all the Unity users who don't know, Unreal Engine 3 and Unity both use the same physics systems, nVidia's PhysX. See here: http://unrealtechnology.com/features.php?ref=physics And here: http://unity3d.com/unity/features/physics nVidia acquired Ageia in 2008.
To be fair all around, there's quite a bit of PhysX that isn't exposed or just doesn't exist in the older version that Unity uses. Soft bodies, Cloth and a lot of other goodies we don't have access to in Unity.
That's true, but all the basics of PhysX are available, and from using the latest and greatest of UE3, I must say that most of the new features aren't all they're cracked up to be. Don't get me wrong they are very cool features, but they aren't something to be thrown around a level willy nilly. There needs to be a careful consideration into performance versus benefit of using the features. They tend to be a performance hog when used without care. Of course that may be all the modifications we made to the engine....but that is another issue altogether.