<p>We have a large MS Access project that requires GUI functionality that is difficult / impossible to do in the VBA / access forms environment. </p>

<p>Over time, we wish to re-write / migrate to a managed C# environment with WPF as the presentation layer.</p>

<p>The only practical way for us to do this is over a longer period of time - converting some of the VBA forms first (that would give the best productivity benefit), and then over the course of 12-24 months convert the entire system.</p>

<p>What do you believe is the best approach. Can MS Access host the WPF forms as we convert them? </p>

What if you look into Visual Studio Tools for Office (VSTO) ?
You can write managed .NET addins for Office in C#, that will integrate as ribbons with office.
Now, the problem is that Access add-ins are not supported in VSTO AFAIK.
But, take a look
here
, it seems to be not that impossible.
So, if you happen to be lucky, you could
Write a C# ribbon for Access which would have all the advanced GUI you need
Write services in C# that could be accessed from Access to encapsulate important business logic for instance, so that you could gradually move not only GUI, but build proper domain object model in C#.
If you decide to try it out, let me know how it turned out :)
Good luck anyway.
PS something else which maybe is helpful?
Access 2007 Developer Extensions (ADE)