Xorg is modular now, Xenocara is the porting effort to bring a "stable" version of the X Windowing environment to OpenBSD.
The version of each component is chosen at the discretion of the developers, which versions are used is documented in the
MODULES file and distributed along with xenocara source tarball, it is also available via CVS.
OpenBSD 4.6 included xf86-video-intel 2.7.1, this is mentioned in the man page as well.. and also indicated in
/var/log/Xorg.0.log if you look carefully.
OpenBSD 4.7 will include version 2.10.0 of that module, perhaps this may include the relevant GMA 3150 changes.. otherwise you can run snapshots and help the developers test things (..like an
important Intel driver update which may wind up in 4.8).
In the future can you add a dmesg in [code][/code] blocks? along with the full non-truncated output of Xorg.0.log.. if you can.