It is the text message symbol with a blue rectangle in the top left of the actual icon on the top of the Samsung Solstice screen. On the bottom left corner of the icon was the draft symbol.
I have that icon too, and I'm not sure, but I think it means the inbox is full(?)
here's a guide I found on Samsung's website:
What Do The Different Icons & Widgets Mean On My AT&T SGH-A887 (Solstice) Phone?=
It's like the 25th item in the list