Used to work fine, does not boot

I have a weird and incredibly frustrating problem.
My Mac used to work perfectly fine (AMD, Zephy 10.5.2 rev2). I usually keep it on for a while, and I can even shut it down without problems. So, after few days of working with it, I shut it down (because I do sleep better with the computer off).
Since then, it will not boot, even in safe mode. I am not able to diagnose the problem either. Also, in between the last two boots, I have not changed anything to the system. No install, no update, no nothing. Just working with text files.

The verbose mode looks roughly like this:

Aoide ReportCrash[17] : Saved crashreport to /Library/Logs/CrashReporter/mDNSResponder_2008-05-11-112312
(many many many more lines about mDNSResponder).

Then "too many crashes in rapid succssion! No crash report being written for pid 552"
Then repeat, repeat, repeat.

I am not sure if MDNSResponder is the problem, or if it is what is supposed to load next after that. Anyhow, tis driving me crazy! Already tried Marvin's utility (does not find anything interesting, since I haven' tupdated anything), disk repair, permissions repair, to no avail..
Any clue?