First up is a minimally functional and cheap screen unit from Sparkfun. This $25 display, when combined with a $15 breakout board and a CPU board, will offer you the ability to show 2 shows of 10 characters.
That's not much to work with, but it would be adequate for a push notification. And as you can see from this demo video shot in 2011, the display is about 3" across so it should be quite readable.
That screen can't do much, but luckily this next one can. About a month ago SEEED released a screen unit which is designed to plug into an Arduino board.
They call it a display shield, and it gets the name because it takes up all the ports on the Arduino board. It costs $50 and is built around a 2.1" E-ink screen with a resolution of 172 by 72.
You'll also need an Arduino board ($25 to $100) and I'm not sure that you'll be able to attach a Bluetooth daughter board at the same time as the E-ink shield.
In fact both of these screen units are going to require a lot of work before they're ready, but if you are planning to go the DIY route I'm sure you knew that.
Me, I'd much rather go for something much simpler. Take this iPhone wrist band, for example. I would get it and use it to carry around an iPod Touch.
Now that is a smart watch. It's also expensive ($35 plus $150 for the iPod Touch), but on the other hand it is much more capable than any other smart watch on the market or off.
I mean, how many smart watches can play Angry Birds? And just think, this watch won't have to be hacked to answer the classic question "can it play Doom?"