There are several exchanges like Kraken, Gatecoin, etc. Is it safe to keep Ether on one of these exchanges when the switchover from Pioneer to Homestead takes place?
Yes, the homestead release won't affect your ether whether stored on an exchange or on your own computer. However, in general storing ether (or any cryptocurrency) on an exchange is always a risk, as the history of various failed exchanges shows (MtGox, Cryptsy). They can be hacked, they can be mismanaged, and they can be fraudulent.
If you want to store and hold ether safely, I'd recommend installing the Mist wallet available here, and backing up the keystore onto a separate device.