(Running latest Snow Leopard and DT Pro 2.0 beta)
I just recently upgraded to Snow Leopard from Leopard. Since then, my WebArchive files wont display in the view window (or partially display eventually) although they are listed. If I open the WebArchive from the Finder (outside of DT) it displays correctly.
Occasionally I will see the text content in the WebArchive, but not the embedded images. Again, if I open the same file from the finder, no problems.
I am having the same problem with DT Pro 2.0pb8 and have not upgraded to Snow Leopard.
I can show the webarchives in Finder, but they will not display in DT Pro. I have to load them in Safari.
Rather disconcerting, since I have so many webarchives in my DT databases.
No problem here displaying WebArchives with public beta 8.
Have you installed a Safari add-on, by any chance? Some of them can damage WebKit, especially if written for an earlier version of the operating system.
To test things out, this sometimes help with DT/webkit or DT/pdfkit issues: Shutting down DT, moving the DT preference plist (~\Library\Preferences\com.devon-technologies.thinkpro2 — or the one for your version) to a safe place, and then restarting DT. This forces DT to rebuild the preference plist. Having done so, you can see if the problem persists.
Warning - you will lose all your custom settings in DT. That is why you move, not delete, the old plist, so you can put it back if needed.
Tried this, no to no effect. Interestingly, I have this problem at home an work on different computers.
I have a few, but cant be sure any are recently installed. I tried removing all 3rd party plug-ins first, then all, and logging out and back in again. The same result each time. Heres a list of my installed plug-ins http://bit.ly/921PKr
Feeling a bit dim-witted. 
After re-installing Safari (to repair webkit), booting into safe mode, moving the plist and generally taking a long journey through negative results, I realized I had yet to do the most obvious… reinstall DT Pro.
All is well and I am red-faced.
(Note: Unlike Andrew, I am running Leopard)
OK… I’ll try a re-install of DT (stay tuned)
I have the exact same problem. Any solutions yet? (i’m using DTPro 2.0 pb8 MacOSX 10.6.2)
Perhaps this Console-Log may help:
Feb 10 21:12:54 Computer DEVONthink Pro[253]: -[WebArchive subresourceForURL:]: unrecognized selector sent to instance 0x9d8ee0
Feb 10 21:12:54 Computer DEVONthink Pro[253]: *** WebKit discarded an uncaught exception in the webView:resource:willSendRequest:redirectResponse:fromDataSource: delegate: <NSInvalidArgumentException> -[WebArchive subresourceForURL:]: unrecognized selector sent to instance 0x9d8ee0
Feb 10 21:12:54 Computer DEVONthink Pro[253]: -[WebArchive subresourceForURL:]: unrecognized selector sent to instance 0x9d8ee0
Feb 10 21:12:54 Computer DEVONthink Pro[253]: *** WebKit discarded an uncaught exception in the webView:resource:willSendRequest:redirectResponse:fromDataSource: delegate: <NSInvalidArgumentException> -[WebArchive subresourceForURL:]: unrecognized selector sent to instance 0x9d8ee0
``` it goes on and on like this...We’ll Ive had no such luck 
Perhaps there are more steps required? All I did a basic re-install of DT (ie. open the latest .DMG file, drag and replace the existing DT in my app folder).
sigh
Also download it againt 
I just got this response from support:
Works well in my case 
Download the disk image again, reinstall the application from the downloaded disk image and everything should be fine.
Yes, thats it. It worked!