This thread discusses the icon:

http://androidforums.com/htc-magic/6652-question-about-notifcation-icon.html

The solution found was:

I found a solution!! Finally

Went to setting>applications>manage applications> SIM Toolkit
Then under that select Force Stop button

after that the sim comes back into action shortly and the notification icon also disappears!
even after a poweroff/poweron remove battery and all.. it doesnt come back

Funny problem, wonder what the force stop does to stop the notification icon coming back?

However, it only seemed to work temporarily.

The notification itself is called stat_notify_sim_toolkit

 

You get the same kind of icon when you mount an SDcard and, as antipesto says, turn it on. This is it 'loading' from the card. Maybe a 256k card isnt fully supported/readable? Like pc wont read all the ram above a certain limit unless you have 64bit?