Logging in to a Mac OS X user account while holding the Shift key prevents Login Items from launching. Similarly, launching OmniWeb with Shift depressed creates a new empty workspace rather than restoring your last browsing session (if so configured). I find this very helpful when I want to switch to a workspace other than the one that was last open or just start up OW quickly.
Broadening the Behavior
The Shift+activate behavioral precedent can be used to form guidelines for two general types of applications.
For applications that depend on a network to provide core functionality, Shift should launch them in offline mode. For example, feed readers would not try to sync and/or refresh feeds, mail and IM clients would not try to connect their accounts.
For document-based applications, if your application can either create a new main window (browser, document, etc.) or restore the last open window set, Shift on launch should toggle the behavior.
OmniWeb itself represents the overlap of these two types. Deciding what to do in such cases is up to the creators of the software. Also, this behavior is not useful in all software, so it isn’t necessary to form rules covering every possible application.
Communicating the Behavior
OmniWeb uses a monolog (it’s not a dialog if there is only one choice!) to inform users that Shift was depressed and therefore a new empty workspace was opened.
Ideally, the affect of the Shift key would be communicated before launch. For launching from the Dock, a modified text label perhaps. The Dock menu command should also change when Shift was held, i.e., OmniWeb’s Open becomes Open with New Empty Workspace.