I've got an excel plug-in in which I spin off a thread to do some work

Dim testThread As New System.Threading.Thread(AddressOf testObject.testMethod)
testThread.Start()
That delegate method attempts to create and show a form:

Private Sub testMethod()
Dim testForm As New Form()
testForm.Size = New System.Drawing.Size(300, 300)
testForm.Text = "Test Form 1"
testForm.ShowInTaskbar = True
testForm.Show()
End Sub
This is pretty simple, and appears to work- the thread starts correctly, and the delegate testMethod is called. However, when the form is "shown", in Excel it briefly blinks onto the screen then dissapears. It doesn't show up on the taskbar, ether.

I have tried the Enabled and Visible properties, and the Focus() and Activate() methods of the System.Windows.Forms.Form testForm object, but none seem to change this behavior.

Does anyone have any ideas for why this would act this way?