If you hold down the option key when you reboot you should see both partitions, though the ubuntu shows up as a "windows" partition.
I use (and highly recommend)
rEFIt, its a boot menu that you can install on your OSX partition, but does no alter any OSX system files. It gives you the ability to select which OS to start up.
I highly recommend reading/perusing the
Ubuntu Forums Its an extremely active and helpful forum geared towards, you guessed it Ubuntu

and there's an apple sub-forum focused on supporting Ubuntu on the Mac