Since Outlook 2007’s Quick Access Toolbar (QAT) uses Alt+ numbers for shortcuts, can you still use Alt+ numbers to enter symbols?

Sure. Hold Alt while you enter the symbol code using the numeric keypad. When you release the Alt key the symbol is entered into the message.

Press (but don’t hold down) Alt then press the number to use QAT shortcuts. When you use the number row across the top of your keyboard, instead of the numeric keypad, you can hold Alt as you press the number.

What are these Alt+ number symbols? See
Using Extended Characters
http://www.outlook-tips.net/archives/2003/20030908.htm