Okay. I Find That I Need To Clarify Something Here. I Find The Bolded Statement Really Wrong. I'll Use Maxis As An Example To Explain Since I'm Using That.
First Of All, We Know That Assisted-GPS Enables Us To Get Satellite Lock Much Faster, Even With Cold Start. Basically, With A-GPS Enabled, Every Time When We Want To Get Satellite Lock, The Phone Will Use The Data Connection (Maxis 3G WAP/Maxis GPRS WAP) To Exchange Information With The Positioning Server. When There Is Data Sent And Received, It Means That We Are Charged By Maxis. For Every 1 kB That We Send And Receive, We Will Be Charged RM 0.01.

So You Said That You Took The Trouble To Conduct An Experiment And The Result Is That There Is No Data Charges When You Use Garmin Mobile XT With A-GPS Enabled. You Are Wrong.
You Might Want To Check That Is Your Assisted-GPS Enabled And Working Properly? To Do That, Go To Menu -> Tools -> Settings -> General -> Positioning -> Positioning Method -> Make Sure Assisted GPS Is Ticked. Then Go Back 1 Step And Choose Positioning Server. Make Sure That You Choose A Valid Access Point (Maxis 3G WAP For My Case Since I Have 3G Activated). To Double Confirm That The Access Point Is Valid, Go To Menu -> Web. Choose Maxis Homepage Bookmark And Select Maxis 3G WAP As Access Point. If The Site Loads, It Means The Access Point Is Valid. Otherwise, You Might Want To Reactivate 3G And Try Again Or Choose Other Access Points. Once You Have Everything In Order, The A-GPS Is Working.
We Will Do A Simple Experiment To Prove That Maxis Does Charge Us When We Are Using Garmin Mobile XT With A-GPS Enabled. Firstly, Launch GMXT. Then Press Agree. If Your Phone Is Facing The Sky, You Should Get Satellite Lock In Less Than 15 Seconds (Green Bars Indicate That You Are Locked With Satellites). Once You Have Gotten A Satellite Lock (If You Are Indoor, Wait For 15 Seconds), Quit GMXT. After That, Go To Menu -> Tools -> Log -> Press Right. You Should See "Maxis 3G WAP" There. More Details On That Entry Shows Roughly 5-6 kB Of Data Transfer. This Indicates There Was An Active Data Connection In The Background When We Are Entering GMXT. In Other Words, We Are Charged For That.
There's Only Data Transfer When You Are Attempting To Lock With Satellites (Remember We Are Using AGPS?), And Usually It Doesn't Exceed 6 kB. Once The Phone Is Locked With Satellites, There'll Be No Data Transfer After That. So If You Are Using GMXT For Tracking, Navigation Or Positioning, There Will Be No Extra Charges. It's Free.
So As A Summary:
GMXT + Without AGPS = Everything Is Free
GMXT + AGPS = Only Pay RM 0.06 For Data To Get Quicker Satellite Lock, Others Remain Free
Use Anything That Requires Data Connection = Pay
As For Garmin Online, Since It Uses Data Connection, It Means You Need To Pay For That. (At Home, I Use Wi-Fi To Retrieve The Weather Info, So Again It's Free For Me).
Have I Made Everything Clear?

Oh My goodness, I forgot to setup positioning server. After I have setup all the need, I logon into GMXT, it require 1-2 min also to locked with satellites but when I go to Logs-Data packet, there are amount that I'm using when I logon GMXT. It is cleared now. Than Bro for clearing it up.