I have an iPhone, but I haven't found a way to limit my time on it. I have found some Applications that will attempt to block you, but they all have their drawbacks (and really, the target people are kids, so adults aren't in mind when it is designed)
I'm essentially looking for the following:
-be able to block all Internet usage from certain time periods during the day. This must block both wifi and 3G.
-As an alternative, blocking access to certain apps (safari, YouTube) would be acceptable as well.
One of the biggest issues is with apple. They really block the developer from doing specific things, such as my alternative suggestion. Of course you can jailbrake the phone, but this doesn't seem to be a common thing users want to do

The tricky thing is saving you from yourself. If there's a back door (ie you can just change the time, remove the program, etc) it's not gonna work.
Is anyone aware of such a thing? Any ideas? I fully understand the root of the problem, but doing it on sheer will power alone is difficult. Sometimes you need an extra boost, and this would be ideal.