To be fair, when the WSJ app first launched on the iPad it took at least a minute to download each issue. They had no 'staged' loading so you had to sit there and watch a progress bar before you could read a single article.
I think its a good idea, there is definitely room for a mock-up / wireframing tool that exists outside of Photoshop or other desktop apps. Unfortunately for you there are several other options on the market right now that (so far) are much better. For example, Balsamiq mock-ups has a web based product that has most of the capabilities of your product with a more polished design (granted its Flash based but not sure thats a differentiator).
I do like the fact that you are doing it in HTML5, and your browser test is pretty ingenious (although would be nice if it wasn't necessary through browser detection).
Keep it up, always room for a better tool for designing!