It's not exactly what you're looking for, but Google Notebook is Google's take at it. (tour)
Also relevant to its merit as a business idea. Your form of a spreadsheet overlay (with a "layer toggle" ?) might be slightly better than Googles. But you're up against a free, quality product from the start.
Heh. This conversation is so common I almost feel like I'm trolling. But it's a very real one today. While we're on it, and I'm getting my fingers loosened up for the day, here's my current thinking about ways to differentiate:
1. Service and Customer Support. If you're not answering all your emails in hours, you're not trying. Customer feedback is Easy Money.
2. Integrating with your customer's ecosystem better. Huge room to grow here these days, with a huge number of different ecosystems. If your app lets me post notes to my Twitter account as I go, while Google only supports Buzz for corporate reasons, and I'm a heavy Twitter, I go with you.
3. Solve a hard, niche problem really well. This doesn't have to be as groundbreaking as you'd think. I know a good number of people who keep talking about upgrading from their Blackberry to a more user friendly phone. But they don't. Because Blackberry, while ugly and unfunctional in my hands, are productivity machines. You know what the feature the last guy I talked about cites? The ability to text message a preset group of people, with easy additions or subtractions. Running a small business, he does this constantly. My Android struggles here. My iPhone did as well. Blackberry figured out what a business user would be doing often, and does all those things well. This allows them to, at the very least, lag behind the bleeding edge in most other areas.