Hi..the same thing happened to my husband's brand new Nexus S as well.. he forgot to charge his phone the previous night and it ran out of battery while at work.. The same black screen with Unlock Bootloader and the lock icon. When he showed it to me in the evening, i connected it to the chager and then 5 mins later removed and re inserted the bttry... And then the phone worked fine.. I thought it is some default screen when the phone runs out of charge

Also sometimes when the power button gets pressed hard it goes into the reboot or recovery option mode.. That is normal right? Or should i be getting the phone replaced?