When you try to use VSTS in project using MCMS2002 you will get this error when you try to checkin any file :

"Unable to cast object of type 'Microsoft.ContentManagement.DeveloperTools.VisualStudio.Documents.TemplateExplorerDocument' to type 'Microsoft.VisualStudio.Shell.Interop.IVsHierarchy'"

to solve this problem do the following:

Custom field types in SharePoint 2007 are pretty cool. They allow you to create your own subclasses of the existing SharePoint field types. You can control pretty much every aspect of the fields behavior and its logic .

To create custom field you must do this steps:

1)      Build your assembly and install it in the GAC on the server or put it in bin folder in your site folder

2)       Copy the ascx control to C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\CONTROLTEMPLATES

3)      Copy the FLDTYPES_Telephone.xml to C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\XML

4)      Reset IIS

"Request for the permission of type
  Microsoft.SharePoint.Security.SharePointPermission,
  Microsoft.SharePoint.Security, Version=11.0.0.0, Culture=neutral,
  PublicKeyToken=71e9bce111e9429c failed"

you can resolve this error by change the trust level in your webconfig file.
You can specify a level of trust that corresponds to a predefined set of permissions for ASP.NET applications. By default, ASP.NET defines the following trust levels:

Full
High
Medium
Low
Minimal
With the exception of the Full trust level, all trust levels grant only partial trust to the application folder of a virtual server instance. For more information on the ASP.NET trust levels

steps to change this security level :
1) local_drive:\Inetpub\wwwroot\wss\VirtualDirectories\yoursite\web.config
2) search for "trust" and change the level to be "Full"
it will look like this
<trust level="Full" originUrl="" />