I have a atack of 3548's connected by gigastack connectors. It never fails that if I power down the stack and power it back up the first ping to another switch in the stack always misses no matter which switch the ping originates.
It misses once then the replies start.
When you power down the switches you clear the arp caches on the switches. When you power them back up and issue the ping command the first ping times out while the switch is arping out for the mac-address. Once the mac-address is entered into the arp cache it will then ping fine.