Off-topic rant: "Someone with spare time on their hands feel like adding a 'Disable splash screen' checkbox to the preferences panel?" That suggestion illustrates a common anti-pattern in open source software. Broken behavior should simply be fixed, not receive an extra option to turn it off. No one wants to have to check an "unbreak my software" checkbox.
I don't think this is necessarily broken behavior. Photoshop has a splash screen too, and from what I understand, it's there because of the long loading time.
A splash screen that can't be minimized, hidden, or covered up with a useful window is definitely broken behavior. I think it can only be implemented by abusing APIs meant for screensavers and full-screen games.
If anyone from GIMP is reading this, I strongly recommend that you fix that or at least allow us to disable the splash screen. Thank you.