RE:
"Can I unpartition a drive without wiping all its data in disk utility?"
Before Leopard, I don't think this could be done (using Disk Utility).
But Disk Utility seems to be different now.
It looks like you can create additional partitions, and also remove existing partitions (at least other than the boot partition).
However, if you choose to remove an existing partition, you'll lose the data that resides on it - same as if you re-initialized.
The workaround is to move whatever you wish to keep from the partition that will be deleted to one that will be retained.