Hey, update here:
Valve has released two offical SketchUp plug-ins along with their Left4Dead SDK.
The plugins also work for other Source games, but you have to own L4D.
One plugin lets you export 3D Models, the other world brushes.
It also lets you assign in-game textures and objects if you simple name them properly.
Here are some tips off the top of my head:
1: To make a hammer-style brush in SketchUp group together a block, or group of polygons (if convex) together. If you make a simple 6-sided block as a group or component you can further edit it in hammer the same as any native hammer brush.
2: Use the texture naming schemes given out by Valve. I have even gone as far as to import all the textures and proper names into the SketchUp library so I can apply them easily.
3: Models are much more flexible, but also require more work depending on your texture use. Models can have single-sided faces, and concave surfaces.
Collision models are easy enough to make, but using a grouping of convex blocks, all with default materials and all 100% smoothed.
4: Ensure you don't have any errors in your model, that is lines that don't meet at a corner, stray lines that don't meet another line, or reversed faces. There are some ruby scripts to help clean this stuff up automatically. I will try to find the links.
5: SketchUp defaults all curves to 12 sides, and all circles to 24. You can change this default number by selecting the tool and typing a number in either right before or after you place your curve. This is the NUMBER one reason for high-poly models in SketchUp.
6: Know the limitations. For example the value brush importer doesn't have any UV or texture scaling numbers, so you have to tweak these in hammer. It however does work well for models.
The followings is all part of a campaign I am working on:
http://www.l4dmods.com/forums/viewtopic.php?f=80&t=3097
Here is a model I made in about an hour using SketchUp and only the native materials inside Left4Dead. The model is 100% brush work with some props added.
Reference photo:
[url=http://www.flickr.com/photos/fredericksburg/2939995054/:1n4q50gn]
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Inside sketchup, and also properly geo-positioned
[url=http://farm3.static.flickr.com/2572/3744142295_35c5c97ea5_o.jpg:1n4q50gn]
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Inside the game engine, with a few quick props added.
[url=http://farm3.static.flickr.com/2467/3744929850_da9074acb2_o.jpg:1n4q50gn]
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This is a much larger building, which currently only exists as a model, and will be converted into 90% brush work, and 10% model detail. Again this model uses only textures that were already in L4D.
Here is a photo of the real building:
[url:1n4q50gn]http://www.flickr.com/photos/mcgervey/3299027241/in/set-72157613789552413/[/url:1n4q50gn]
[url=http://farm4.static.flickr.com/3593/3600025498_293a9943df_o.jpg:1n4q50gn]
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[url=http://farm4.static.flickr.com/3642/3604975195_70cd83aa97_o.jpg:1n4q50gn]
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[url=http://farm3.static.flickr.com/2434/3663320760_137d4e66f3_o.jpg:1n4q50gn]
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[url=http://farm4.static.flickr.com/3654/3662518771_0c17d37249_o.jpg:1n4q50gn]
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