I've been an exceedingly annoying problem on my iPhone since 2.1.
Every few hours, most applications start crashing on launch. It's always a random subset — Mail, Cydia, iPod, Safari and others will crash, but I can launch BossPrefs and reboot.
Post reboot, I notice Cydia, Settings, and Maps crashing... and BossPrefs too. I'll usually only try a few before restarting, but at least half of applications may become affected at any given time.
Respringing never helps. After a reboot, Mobile Substrate sometimes goes into safe mode, and sometimes the entire system freezes for several minutes at the lock screen before I can start using it.
After four or five reboots the system comes up normally... and the crashes start again after a few hours.
I have restored multiple times, rebuilt my IPSW in PwnageTool, and restored without restoring from backup.
I've even observed Cydia crashing ON THE FIRST BOOT AFTER A RESTORE, before iTunes has even seen the device.
WTH?! Anyone else seen this?