Nowadays virtual environment are very common, thus a hot increase of an existing drive into a virtual machine is a very common operation (roughly equivalent to a LUN expansion in SAN).
When you expand a zpool device, the new size won’t be immediately available to zfs, but you can easily request an expand with the following:
zpool online -e pool device
That will immediately grow the pool using the newly allocated space.
For further details check out this page.
Update 06/2014: while the above procedure is related to Solaris, some time ago I was trying to use it to expand a zpool on FreeBSD where I’ve found that the above procedure won’t work. This happens because first you need to grow the partition where the filesystem resides and then grow the pool itself. I’ve found this useful article that describes the FreeBSD procedure.