When Aptitude's bottom status line permanently states
[1(0)/...] Resolving dependencies
(i.e. Aptitude can't find a solution given the current constraints), changing the status of any package (e.g. telling it to install a different version) results in a multi-second period (depending on the computer's speed) during which the screen is frozen while aptitude tries to find a new resolution.
Is there any way to convince aptitude to run its search for a solution either in the background or not at all?