I read about this iPad keyboard trick recently.
If you drag from the "123" button on the keyboard, it will temporarily show the keyboard for numbers and symbols. Move your finger to the key you want and release. The character will be typed and the first keyboard will return.
The iPhone keyboard has a similar trick that when you drag from, say, the key for the exclamation point and comma. Drag from it and you can quickly type an apostrophe. Many keys do this.
I practiced with b$str0ngm@n for a few seconds and I think I could get used to typing it on a regular basis with a minimal hit in speed.
Actually, the plus symbol has nothing to do with in-app purchases. It denotes universal apps -- apps which will run on the iPhone or iPod Touch as well as the iPad.