[Tarl the Wicked]

I've been a DM off and on for several years. In fact, my sign is a charicter from a game I used to play in (A Lawful/Evil Priest of Set, boy what fun!)
I currently run a game that has charicters from about 8 years of campaigning from the group. I have incorporated many aspects of Planescape and Plane hoping is common.
Now, that said,
Many of my players have extremely powerful charicters, but regardless of how powerful a charicter is always remember that on his home Plane, a God is, well, a God/ you can't Kill him.
I often put my players in very dangerous positions. It is not unusual to have a party lose members to being captured or killed. (Many spells and abilities will keep your player from losing the charicter, but once he is gone from the adventure I do not allow him to return). All my players have multipul charicters. If the goup they are running falls/is captured/incapacitated, they can always come to rescue them with another group.
On the point of trying to kill your players charicters. If you really wanted to it would not be to hard, but set up my monsters/encouters for the high level group as if THEY are the monsters and I want my charicters to survive. This makes your monsters tough without being overpowering.
On the subject of the rules being guide lines, I agree. This can also apply to the monster manuals. Just becaus it says Orcs are 1 hit die, does not mean that there are not 10 or even 100 hit die Orcs out there. Also, playing in the pplanes, keep in mind that the higher level Demons and Devils from the manuals can be 'types' or 'classes', not single individuals.
Finally, I always give my players an out. Most of my adventures can be solved with or without violence. If the players really work at it they can get the scroll from the lich king without killing him.