Two ideas:
One is to use a calendar application, and simply put in whatever you did with the date, time range, and a description. I will be releasing one soon, I hope, but any generic calendar app should do that, including Apple's. By using the category field, you can group certain events similarly, search, etc.
A second crude idea is to just use a datestamp on a Notes page. That basically gives you a big long text document, easily searchable, etc. When you say "reports" do you mean a diary, or something more?
I think I am confused by what you "really" want, though. Doesn't the to-do application you mentioned keep track of what you did, too? If you can't make an entry for everything you did in the to-do, then how will you enter the data somewhere else to generate the report? At some point you need to input the data.
My gut feeling is that you want is a good Calendar application, then you can make recurring entries, mark them as to-do, track time blocks, etc., and (perhaps) be connected with Contacts. It doesn't makes sense to be using two programs (To-do and diary) when one can do everything. Unfortunately, we are developing for a computer, not for the iPhone. But again, I'm not sure what reports you are trying to generate. Getting reports from an iPhone app certainly sounds like a brave new frontier. We wouldn't be building a sync to an iPhone until 2.0, I suspect. Kicking out an importable to-do list would probably be easy, though.
This is a sneak peak of the edit window. Would this capture all the data you need?
