Thanks Justin for the tip (renaming plugin folder and retrying login). In my case it turned out to be the Bad Behaviour (v2.0.7) plugin.
My problem: WordPress v1.5.2 Login stopped working suddenly after 1.5 years without problems. No new plugins or code changes had been made. wp-login.php would just hang in an infinite loop and eventually timed out or my browser would offer me the actual wp-login.php file to open or save.
Solution: Renamed the /wp-content/plugins folder. Retried login. No problem this time. So I turned off plugins one-by-one until login worked correctly again. Turned out to be Bad Behavior v.2.0.7 was ‘behaving badly’. Upgraded to Bad Behavior v.2.0.10 and everything is now hunky-dory.
