Seems that Stippi (Stephan) updated WebPositive to version 515 for Alpha 2. This version will not work on newer, Haiku trunk builds - use other version linked below for that. Instructions below how to download it. Rename your old WebPositive folder to avoid losing it.
http://haiku-files.org/files/optional-packages/WebPositive-r1a2-gcc4-x86-r515-2010-05-22.zip
unzip to /boot/ for normal install.
Thanks!
mmlr site is down!
Your package is broken
Missing symbol: _ZNB9Language…
r36814 gcc4hybrid
The linked WebPositive build above is only for Alpha 2 Release. 
You need to download another version for Haiku Trunk Builds. Two versions of WebPositive, one for Alpha 2 (and much older Haiku trunk builds) and one for newer Haiku trunk builds.
For newer, Haiku Trunk Builds:
http://www.haiku-files.org/files/optional-packages/WebPositive-gcc4-x86-r515-2010-05-22.zip
Nice, it’s still just getting better! A few things are still missing, but it’s certainly looking good.
installed r515 on Haiku Alpha2, no problems. surfing wireless from bed, on an IBM laptop, with a native webkit based browser in Haiku. wow. Haiku IS progressing nicely.
Suggestion for the next version: WebM support !
Opera, Chrome and Mozilla already got it, Explorer soon.
+1! It should probably be implemented through the media_server though.