With the release of Visio 2010 you have the choice to install either the 32bit version or the 64bit version. What version you are able to install depends on:
Visual Studio Tools for Office (VSTO)
If you are using VSTO as the architecture for your Visio based add-ins you are in luck. These add-ins are based on managed code (C#, VB.NET, etc) and compile to MSIL, meaning that when the VSTO runtime executes a VSTO add-in, the .NET runtime compiles the MSIL to native code based on the target platform. In other words, if your add-in is VSTO based then there are no changes that you have to make to your code in order to support both 32 bit and 64 bit versions of Visio 2010.
Anything other than VSTO (VSL, COM add-in, etc.)
If you are not using VSTO as the architecture for your Visio based add-ins then you will want to build both 32 bit and 64 bit versions of your add-ins.
For additional information read this article which explains 32 bit / 64 bit issues in Office 2010 Compatibility Between the 32-bit and 64-bit Versions of Office 2010
FYI – this article also discusses changes to VBA for the 64 bit environment.