I've got loads of their bulbs around the house as I'm swapping all my lights from Hive to Tradfri. One of the things missing from the Tradfri platform is the ability to turn a light on at sunset - you can only set an absolute time.
So, this morning, I thought I'd have a play (I'm very new to Python so this isn't intuitive). Couple of hours later and I've got a little Python script that gets the sunset time, compares it with the current time and switches the lights on if it's later than sunset. I'm running some other home automation stuff with a script that runs every few minutes so it'll be dead easy to add this in.
For the record, as it's in the spirit of "hacking" home automation, my pet project, which is what got me started in this, is occupancy detection so that I can control the heating depending on who's in. I've now got that 99% working. There's a Pi in the lounge that sniffs for the Bluetooth MAC addresses of the phones in the house. If it finds one, it increments a score. If the score is zero then it turns the heating down. I've even got it weighted so that if I'm in the house it turns the heating up a bit more. I'm using the very badly documented Hive API to do this so it's been a bit of a labour of love. When I started this, I knew nothing about Python or APIs, or much else that would help with this. I don't do "Hello World!" so this has definitely been a bit of a baptism of fire for me.
This is the rough way that it works:

I've done a bit of a write up here:
http://2610.co.uk/blog/hivehome-geofencing/
Lots more to do on it. For starters, I can now add in the functionality on the dashboard to turn my Ikea lights on and off!