Well, since PR1.2, Maemo gets into an infinite reboot loop when it can't mount /home. The method involves moving /home. If you mess up the entries in genfstab.awk and/or rcs_late, or maybe the copy of the home directory, it could be that Maemo can't mount home, resulting in a 'bricked' device.

Though bricked isn't the correct word here, as the phone can be revived by reflashing it. So no: it can't be bricked and yes: if something goes wrong, you'll have to reflash*

I don't know about your linux knowledge, but if you understand the commands involved, it should be safe to execute them.

*While trying out different partition configurations, I've messed up once too, causing Maemo to be unable to mount /home resulting in a reboot loop, but a reflash solved this.

Edit: I've added an extra warning to the wiki entry