Make your plants twitter when they get thirsty.
At MakerShed you can buy a kit to enable the twitter skills of your plants and flowers.
I liked the idea until I saw the price tag, 100 usd per pot. And for that, you still have to do the watering manually. I'm not sure I would like to hear my plants desperate cry for help when I'm away for a few days. But on the other side, the thank you message after they get some attention is appealing.
In the end I think I would opt in for a less geeky watering system for about 40 usd. And perhaps build myself a monitoring system that spits out some nice charts to make sure its all working.