In Leopard's Disk Utility, yes.
What you can do 'live' (IOW without reformatting) is:
• decrease the size of a partition (of course not to less than the space occupied)
• add a partition (if there is some free space)
• increase the size of a partition if the adjacent space is free. So for example if you partitions A, B, and C and you delete partition C you can increase the size of B to cover what was previously C.
What you can't do in DU is shift entire partitions around, IOW you can't modify where a partition starts.