Why would you need an online service to do that, if the point is just to write code for you that you're not going to share with anyone? Or did I miss something?
(I don't understand the point of 750words.com either, for that matter, except if you're not satisfied with a text file and quick scripts for reminders and word counts: but if you want to do a diary like 750words.com about coding and getting your hands dirty, why wouldn't people just code the thing they want?)
You don't need a service like that; just like you don't need 750 words to write three pages a day; and you don't need facebook to stay in contact with your friends.
This isn't about need - most little one off ideas like this aren't about that. First, it's about encouraging the idea (keep the hand moving/practice a kata) via a nice, simple online tool. Second, it's a central, online service - ala google docs, drop box, etc where you could, every day, just upload a snippet of code you toyed with.
So; no - you and I don't need an online service to do this. No one does. No one needs an online calendar or online service that hosts a private journal. This is more about the idea and encouraging that idea and practice than making a profitable service millions flock to.
Make a nice little tool to share the idea, and a service that encourages the idea. Sure, someone might code the thing they want to make (e.g. the app/service itself) but if the original idea helped spur them to do so, then I've happily succeeded in planting the idea in someone's head, and hopefully they will also do so in turn.
Or they could use posterous, livejournal, blogger - or an internal text file. There's a million things that could do it, the idea is to make one tailored for it and tailored to the idea. Sometimes a custom built system can beat a generalized system made to "sorta do it".
(I don't understand the point of 750words.com either, for that matter, except if you're not satisfied with a text file and quick scripts for reminders and word counts: but if you want to do a diary like 750words.com about coding and getting your hands dirty, why wouldn't people just code the thing they want?)