Ok got it figured out I think. The tab stops are measured in 'twips' (1/1440 an inch), so I wrote a quick function to convert inches to twips and added it to your example. You will notice the letters are now tabbed out at 1 and 4 inches.

 Code:
Break On

$t = "{"+
     "\rtf1\ansi\ansicpg1252\deff0\deflang3082"+
     "{\fonttbl{\f0\fnil\fcharset0 Times New Roman;}}"+
     "\viewkind4\uc1\pard\tx"+InchesToTwips(1)+"\tx"+InchesToTwips(4)+
     "\f0\fs20 A\tab B\tab C\par D\tab E\tab F"+
     "}"

$S = CreateObject("Kixforms.System")

$R = $S.RichTextBox()
$R.Width = 400
$R.AcceptsTab = 1
$R.Rtf = $t

$F = $S.Form()
$F.Width = 420
$nul = $F.Controls.Add($R)

$F.Show()
While $F.Visible
   $nul = Execute($F.DoEvents())
Loop
Exit 0


Function InchesToTwips($inches)
   $InchesToTwips = CDbl($inches)*1440
EndFunction

Edited by ShaneEP (2013-02-02 09:20 PM)