I'm currently playing with an Arduino, I'm only at the traffic lights stage, which feels very "Hello World", but I enjoy it. It's very refreshing to think that I can start building my own hardware to interface with a computer. I've spent years writing software and it makes a nice change to make something tangible using the programming skills I already have.

I agree, the colour changing lamp isn't that useful by itself but learning how to make one would be fun. It's a nice hobby to have. There's something satisfying about building something yourself.

The limitations seem endless, but it's coming up with practical ideas that I think is the really tricky thing.

Stuff like the Baker Tweet - http://www.bakertweet.com/ have a lot of potential.