Mac Book Pro 3,1 uneven startup behavior - sometimes start between 5min to 30min after pushing power button
Once you've gotten your backup - get into Disk Utility and try to repair the disk / consider an erase install. It will likely fail (unless the delay was caused by a very unlikely file system corruption or startup conflict).

Dropping a clean install - only the OS will let you know right away if the Hard Drive (HDD) can take new data or if other hardware issues are causing the half hour delay. Something fairly destructive is needed to cause a mac to take that long to successfully boot. Take it to a hardware tech if you have any doubts how to troubleshoot this, but odds are it's storage related.