I was recently doing some InfoPath 2007 development in Visual Studio 2008 (sp1) for a client. The form had a number of views, collected several hundred data elements, and contained a large number of data connections. Over time (as the form grew larger), I began experiencing an error where, whenever I tried to open the InfoPath form (.xsf) in design mode, a modal dialog was triggered. Unfortunately, the dialog was hidden, so the form would not display and a variety of Visual Studio controls (such as the menu and toolbar) would be locked. When I tried to close VS, I got an error message stating that VS could not close because a modal dialog was active. The only way out was to kill VS through the Task Manager. I fought with this for a few hours before I was able to track down the cause (sort of) and a workaround.
The underlying issue appears to be a timeout when the form tries to verify on the server (on form load). Clearing the "Verify on Server" option in the "Designer Checker" tab resolves the issue. In my case, I couldn’t get the form to load in design view, so I couldn’t get to the "Designer Checker" tab. As it turns out, the "Verify on Server" option is a yes/no attribute that can be changed in the .xsf file by opening it with an XML editor and searching for "verifyOnServer". Flip the value from "yes" to "no", save and close the .xsf, then re-open the .xsf in design view. Hope that saves you some time and frustration.