With the 1.5 patch the Witcher no longer requires a disc check among other fixes and community content.
"Patch 1.5 now available!
We are happy to inform that latest patch version for The Witcher is now available to download!
Here are the features of patch 1.5:
Modifications:
- copy securities (DRM) are removed
- playing the game no longer requires the game disc in the drive
- patch solves problems with EAX and blurred graphics
Additionally:
- 5 bonus adventures made by the Witcher community: ‘Deceits’, ‘Blight of the Bogs’, ‘Wraiths of Quiet Hamlet’, ‘The Wedding’, ‘Merry Witchmas’.
In order to install patch 1.5 correctly, original Witcher game upgraded to 1.4 (i.e. The Witcher: Enhanced Edition) should be installed.
You might also want to download the new game-difficulty modification, "Flash Mod", prepared by our community member Andrzej "Flash" Kwiatkowski. Now surivivng in the world of The Witcher will be much more challenging than ever before!"

Is this patch on Steam yet?

Does it work with other download versions (i.e. not Steam)?

If they were compatible with previous patches they will be compatible with this one as well. Direct2Drive often requires its own patches, but this is less common with GamersGate and the like.

I don't see an update for the steam version on the front page. I'm using the retail one so can't comment on Direct 2 Drive or the others.

Just to confirm, I tried the patch on my copy purchased directly from Atari, not through any of the third party download services, and it seems to have worked fine. Not sure if the DRM removal really applied to this version though, since it was obviously already diskless, but did require activation.
EDIT - I take part of that back. I don't seem to have the "Blight of the Bogs" community created adventure that was supposed to be one of the 5 new ones added.

woot i'll download it soon

Nope, and looks like retail patch is not working too. CD Projekt RED and Valve is not get along together well I think.
http://forums.steampowered.com/forums/showthread.php?t=914113

How dare they support this old game? Do they feel no shame? At least Bethesda is professional enough to charge 10 $ for that kind of stuff.

Valve doesn't want other developers supporting THEIR old games for free now?

No, CD Projekt RED doesn't care about uptading Witcher on Steam. They released EAX hotfix on Steam one month after the Retail patch.

Bit hard to know which is it: either CDP gives the patch to Steam who takes its sweet time to apply it, either CDP takes a long time to deliver it to Steam. I know Steam is evil but CDP is a bunch of commie Poles so they both have their faults.

Lulz. CDP Red is the developers of The Witcher. CDP is the owner of CDP Red and GoG. But it's Steam that takes their time. They have to prepare their flaky overlay thingies, DRM, and prepare the servers for distribution and such. I reckon it will arrive about the time the all planets are aligned.

I also heard that CDP partakes into satanist rites on a daily basis.