Hi:
 
First the disclaimers - Use this at your own risk.  I have done testing but you may run into problems so make sure you are backed up appropriately.  I can't really support this but I may be able to answer a few questions on the forum.
 
I am running Windows XP Pro, Outlook 2003 SP3, GTD Addin 2.7.5.  On the BB side I have a Verizon 8703 with OS 4.1.0.377 and NextAction! 2.0 just purchased.    My company uses Microsoft Exchange and all my syncing is done wirelessly.
 
I must acknowledge Sue Mosher's amazing www.outlookcode.com  and the gtdsupport.netcentrics.com/forum member Ken Glade.  Both sites had tons of information that helped me slog through this.
 
I have written an Outlook macro which will create NextAction! Projects/Contexts from GTD Projects/Actions and vice versa.  Hence you can create tasks either in Outlook with GTD or in NextAction!.
 
Usage:
 
Outlook creation - Create a task in Outlook, and select the Project and Action choices.  When you are finished, run the macro.  This will create a category for that Task in the format of p:ProjectName.  It will also create another category that matches the Action ( @Calls, etc . . . ).  The macro also saves and closes the task.  Each task will have two categories.
 
When you sync your Blackberry, the task will be available via NextAction! with a Project called p:ProjectName and a Context which matches the Action from GTD ( @Calls, etc . . . ).   NextAction! requires the p:ProjectName format.
 
NextAction! creation - Create a task in NextAction ( or in the normal Tasks, but open it in NextAction! ).  Select the Context and Project and Save the Task.
 
When you sync the BB and open Outlook, you will see Tasks with No Project and No Action, but with two categories.  Open them one at a time, and run the macro.  The macro will determine that the Project and Action field do not exist, and will create them from the Project and Context that came from NextAction!   The macro will strip out the p: from the beginning of the Project for GTD.  The macro also saves and closes the task.
 
I think the easiest way to do this ( if you already use the GTD Addin ) is to go through your tasks in Outlook, open them one at a time and run the macro for each.  That will convert your Outlook Projects to the NextAction format, so they will be available in NextAction! and you don't have to create them on the BB.
 
I put a Toolbar button on the Tasks form so I can run the macro with a click. You can do this by right clicking on the toolbar > Customize >  Commands > Macros > drag the macro button onto the Task form.
 
Here is the macro code:  You have to open Visual Basic editor in Outlook and Insert a module.  Paste this code into the module and Save.  It will then be available to Run or as a Toolbar button:
 
Good luck, and get something done for a change!
 
Sub Sync_GTD()
 
Set myOlApp = CreateObject("Outlook.Application")
Set myItem = myOlApp.ActiveInspector.CurrentItem
Dim GTD_Project As Outlook.UserProperty
Set GTD_Project = myItem.UserProperties.Find("Project")
Dim GTD_Action As Outlook.UserProperty
Set GTD_Action = myItem.UserProperties.Find("Action")
Dim Cat_List As Variant
       
If TypeName(GTD_Project) <> "Nothing" Then
'                                           This means GTD Project is present
    Next_Project = "p:" & myItem.UserProperties("Project")
    Next_Action = myItem.UserProperties("Action")
    Cat_List = Array(Next_Action, Next_Project)
    myItem.Categories = Join(Cat_List, ",")
   
myItem.Close olSave
Exit Sub
 
  Else
 
  arr = Split(myItem.Categories, ",")
'                                                   arr is the array of Categories
   If UBound(arr) >= 0 Then
'                                                   item has categories, Assume two categories from Next Action
'                                                   For I = 0 To UBound(arr)
'
         If InStr(1, Trim(arr(0)), "p:") > 0 Then
'                                                   The Project will have p:
            ProjName = Mid(Trim(arr(0)), 3)
            ActionName = Trim(arr(1))
         Else
            ProjName = Mid(Trim(arr(1)), 3)
            ActionName = Trim(arr(0))
        End If
 
    End If
   
Set GTDProj = myItem.UserProperties.Add("Project", olText)
GTDProj.Value = ProjName
 
Set GTDAction = myItem.UserProperties.Add("Action", olText)
GTDAction.Value = ActionName
 
End If
 
myItem.Close olSave
 
End Sub