That's a good question. The answer is NO. DirectAccess is actually a feature that comes with Windows Server 2008 R2 and Windows 7 (Enterprise and Ultimate editions). You can get DirectAccess working right out of the box if you have these operating systems in place.
In fact, you might want to try the Windows DirectAccess solution before you go to UAG DirectAccess. UAG does add some cost, so you might want to try out DirectAccess on your network so that you'll be convinced that DirectAccess is the key technology that is going to make you and everyone else in your organization more productive.
However, if you go this route, you have to be aware of some of the limiatations of the Windows DirectAccess:
* First, there's really not a viable high availability option for the Windows only DirectAccess *Second, your DirectAccess clients will only be able to reach resources on Windows Server 2008 above servers, and the applications must be IPv6 aware
If you can deal with those limitations when testing, then go for it! Then when you're convinced that it's the greatest thing since sliced bread, you can moved up to the UAG DirectAccess solution. However, keep in mind that you'll have to redeploy, since there really isn't a migrration path from Windows DA and UAG DA.