i would like to show you a very cool feature in Sketchup, that is extremely useful in architectural modeling or achiviz, and would be awesome to have it in blender 2.5 or 2.6
can this be done in some way?
Yes its possible but not like Sketchup does it (non-destructive)
Here is how I do it maybe this will help:
1 Build you model to completion like you normally would
2 Copy all the parts that you would like in the section cut to a separate layer
3 apply any modifiers that would get in the way like Subsurf, Mirror or Solidify.
4 Then join all of the copied parts with alt+j
5 go into edit mode and select all (“a” untill everyhting is highlited)
6 hold “K” while you draw a line along the cut line
7 select the new cut loop you just created and scale to zero along the relevant axes. In the case attached image 2 for me it was along the X axis
8 delete the faces that you wish to eliminate see image 2
9 high-lite the new open areas and do beauty fill (alt “f”)
you now have a cut away. This is by no means the best method, but it works for me untill they add the real thing to blender (maybe a modifier?).
Attachments
thanx a lot Greyoxide.
where should we ask for such modifier to section non-destructively?
you can always use in viewport the equivalent of Alt-B to create a section like in 2.49
you could also use node Z depth feature which can show section also but more difficult to do!
in 2.49 there is a script to do a cross section of a model in 3D
but this really show a cross section not a part of the model = to a plane cross setion
salutations
Ricky, could you please reformulate the post above, so can a nook like me understand what you are talking about? 
We might be able to help you more if you tell us what it is your trying to get a cross section of.
Rickyblender has some good ideas, but they would cause a lot of limitations in an architectural render. Especially if you are using an external render.
Can you upload a copy of your model, or maybe some screen-grabs?
thanx guys,
i want to do the sections how is on the youtube video, excepting for the animation. But the animation would be good too 
You could use booleans but they can sometimes be buggy and slow.
http://www.screencast.com/users/blenderwho/folders/Jing/media/484bf364-e7c5-4e7c-9a43-c50e32548a69
did you ever use the Alt B function to cut a scene in viewport before ?
just do Alt B then make a square in the viewport you’ll see what i mean
do it over some object to see the effect
then to come back you redo Alt-B again
this is in 2.49
hope you understand what i mean here!
happy 2.5
i have 2.5 , and alt+b does nothing :-??
thanks it works.
but this is not even close of the power of sketchup :((
actually is not bad i played a little more and it’s a pretty nice feature.
too bad it cannot be exported as a dxf with the lines seen on Clipping Border 
well you can always make a snapp or save the screen but your right it does not show as such in render!
now may be you can use the Node Z buffer clipping feature
but a little difficult to adjsut but it will show in render!
check out the nodes wiki page for this composite effect!
salutations
That would be an interesting addon. How hard would it be to capture the clipped geometry? That way you could isolate a section for export or render.