Actually, you can
Uwe Hermann and Russell Coker talked about resizing ext3 volumes in several scenarios. While they point out interesting things (i.e., the fact that you can resize stuff without losing data), they both get the important bits wrong.
Uwe, you can do online resizing. Just use
resize2fs <device-node>. The kernel will notice that the device is mounted, and perform an online resize. This takes slightly longer than an offline resize, but (a) you don't need to do the fsck, and (b) you can still write and otherwise use the file system while the resize is in progress, so the net result is that you win time, especially if the file system in question is huge (I've done things like resize a 700+G LVM volume by 50-100G, which takes half a minute or so; an fsck will take seriously longer).
Russell, resizing the LVM volume on which the root filesystem lives used to deadlock the kernel in the old days, but that bug has been fixed since around Linux 2.6.13. I've done online resizes of the LVM volume (and ext3 filesystem) of my root filesystem many times since then, both on some of my home systems and on huge customer boxes, and never encountered any problems.
I really love LVM, especially on servers. /home is full? lvresize -L +5G /dev/<hostname>/home && resize2fs /dev/<hostname>/home, and we can work again.