You can also sweat a threaded end onto the hot water pipe and then use
a flexible copper piece with threaded fittings and rubber gaskets on
each end in place of hard soldering the pipe to the tank.

See the following:

http://cgi.ebay.com/Watts-J-Flex-Wat...QQcmdZViewItem

That is what I used to connect our water heater to the existing copper
plumbing, it works great.