FYI Non-portalable textures
any tanslucent/parent textures are unportable, and make the partifcle spread effect. i used a bunch in my map.
{
"$basetexture" "Metal/metalwall_bts_002a"
"$surfaceprop" "metal"
"%keywords" "portal"
"%noportal" 1
...more stuff...
}
The line %noportal is either 1 or 0 depending on weather you can put a portal on the material. Reading Valve's development wiki, it says that lines starting with a % are directives for the tools only, and are not loaded by the game. From this we can assume that blocking portals is done somewhere in the toolchain, either in Hammer or most likely the new BSP program, once we have access to the updated SDK it should happen to our maps as well.
Spot on mate. If you want to do it the hard way, so be it. The easy way is what Ralph suggested, you just have to wait.
I worked out an easier way, maybe not the best way, but it works.
Any brushes you want unportable, tie them to a 'func_door'
A portal can't be created on a func_door brush. (make sure you deselect the 'on touch' flag, otherwise your walls are gonna start moving all over the place.)
I'm not sure what other func entities would work, i'm also not sure how make a whole alot of func_door brushes will effect map size.
EDIT: there a problem tough, the brushes don't show stuff like reflections that well.
The func noportal_volume brush will do for the mean time, when the new SDK comes out you can just go into view all entities and remove all of them, as simple as that.
WARNING: Don't be lazy and tie all of the brushes to portal_novolume. I thought I Was really clever and tried to do that, and when I loaded up my map all of them are nodraw by default! I guess we really have to deal with large map sizes untill the real SDK comes out.
What is taking them so long..
Btw, we need a cake and companion cube emote.
[img]www31.brinkster.com/taob1/Signiture2.bmp[/img]
I guess I'll make a cake one later tonight. Until then, have some turret. 
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"Your skill in Forum Quickdraw has increased by 1."
No, I accidently pressed the "edit" button since it is right next to the "quote" button and it took me a second to figure out where my post went, since it was never there.
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Oh well.