Download uImage version you want to try.
Adjust /boot/uImage link to point to downloaded uImage-2.6.xx.....
Download modules*.tgz for the same version of course.
Untar modules*.tgz in / (it will create /lib/modules/2.6.xx)
Call depmod -a VERSION (where VERSION is the same as name of directory in /lib/modules created from modules*.tgz)
Kudos to Thomas White, Lars-Peter Clausen, Paul Fertser, Thibaut Girka, Gennady Kupava and other kernel hackers for their great work!

All changes from switch-for-kernel are integrated in new SHR-U images which also contains latest 2.6.32 kernel from here. You can still try 2.6.34 kernels if you want without any changes to userspace.
Sound won't probably work in phone calls, ask dos1 on #openmoko-cdevel to publish diff for alsa scenarios

Deprecated info:
To test 2.6.37 kernel you have to switch back to evdev driver 'sed -i "s/tslib/evdev/g" /etc/X11/xorg.conf'
Or use patched tslib http://permalink.gmane.org/gmane.comp.handhelds.openembedded/41363
For 2.6.32 you need longer rootdelay/rootwait (rootdelay=7 works for me, or rootwait without value) - adjust in u-boot environment or in /boot/append-GTA02 for Qi, booting with Qi while POWER button pressed (debug) works too. Last versions boot ok when I remove usb cable before booting even with rootdelay=1.
For 2.6.3* with GSM, GPS, sound you need to patch FSO a bit, see this script. Please read what it does and then run it with right param.
With u-boot and >2MB uImage you need to increase how much nand u-boot should read from kernel partition see this wiki entry.
For uImage-2.6.32.11-oe1+gitr5+14be1091928fcce66812a85129768fb253b36420-r7-om-gta02.bin and newer e17 wont start if you have touchscreen as /dev/input/event4. switch-for-kernel should update it, but all those changes are overwritten when opkg is upgrading those files, so be carefull.
If you find new issue not reported on http://docs.openmoko.org/trac/, please report it there, not reported issues cannot be fixed. Most annoying issue right now is #2328 (fixed in oe3) and #2337 (fixed in oe4).
2.6.34 does not support touchscreen filtering in kernel driver. We need to adapt something like evfilter. Without filtering you often get short drag event instead of single click even while using stylus! You can try it in e17 illume settings dialog, when only not-scrollable "Close" button reacts to single click and all other items (scrollable) needs double click (or really precise single click).
All kernels here (except 2.6.34-oe1 now) have DRM patches from Thomas White (How DRI and DRM Work)
Versions before uImage-2.6.32.9-oe1+gitr5+14be1091928fcce66812a85129768fb253b36420-r2-om-gta02.bin and are known to fail to suspend/resume (WSOD). But KMS and DRM is working without any change needed to shr images.