Today I did it. I’ve upgraded my system from Intrepid Ibex(8.10) onto Jaunty Jackalope (9.04). My Ubuntu x86_64 installation offered me, as always, a couple of new packages for update, and as I wanted to update, a message box told me, I could do a whole system upgrade onto Jaunty Jackalope. So I did and it worked. I took a very long time (>1h), but I had no unpleasant surprises afterwards.
All special sources.conf entries in my apt-get config have been commented out, and I started from scratch using a new Ubuntu installation.

The first thing I checked out was of course the VDPAU functionality of the new Nvidia driver version 180.44. My focus was on tearing and resource problems. I also was curious whether the usage of TwinView (dual headed) is now possible together with VDPAU.
I’ve checked it with Compiz and Kwin.
My tests were made with a AVCHD sample which I’ve made with my Canon HF100. The line I’ve been using for playback was:

/usr/bin/mplayer -fps 50 -vo vdpau -vc ffh264vdpau  my-h264test-sample.mts

Here’re my results:
Using Compiz and Kwin with “Enable desktop effects” always gives this:

VO: [vdpau] 1920x1080 => 1920x1080 H.264 VDPAU acceleration
[vdpau] Failed creating VDPAU decoder: The system does not have enough resources to complete the requested operation at this time.
FATAL: Cannot initialize video driver.
[h264_vdpau @ 0xcb53c0]get_buffer() failed (-1 0 0 (nil))
[h264_vdpau @ 0xcb53c0]decode_slice_header error
[h264_vdpau @ 0xcb53c0]no frame!
Error while decoding frame!

FATAL: Could not initialize video filters (-vf) or video output (-vo).

So I hope the experimental driver 185.19 really fix this problem, because it is told to work almost with every RAM configuration. Mine is a Nvidia GeForce 8500GT with 256MB.

Going fullscreen by pressing “f” while the latest mplayer (SVN-r29238-4.3.3) is playing back my test sample, results in ca. 1 of 5 runs in a crash:

[vdpau] Error when calling vdp_presentation_queue_block_until_surface_idle: An invalid handle value was provided.
[vdpau] Error when calling vdp_video_mixer_render: An invalid handle value was provided.
[vdpau] Error when calling vdp_presentation_queue_display: An invalid handle value was provided.
[vdpau] Failed VDPAU decoder rendering: An invalid handle value was provided.

The screen freezes and a green strip/line at the bottom appears. I though this bug have’ve been ironed out already. Well.

Tearing is still present. No matter which settings I use. I’ve tried VBlank and the “No Composite” workaround in the xorg.conf ,

Section "Extensions"
    Option         "Composite" "Disable"
EndSection

but this works only in a single head (no TwinView) constellation.
But: TwinView in general seams to work now reliably.
To sum it up: The working constellation requires the absence of a compositing window manager. So Compiz still doesn’t work for me with 180.44 and in KWin (KDE) one has to turn off the desktop effects. I guess for Metacity (Gnome) this is also true. To get rid of tearing you have to turn off TwinView. And if this doesn’t help, disable the compositing in the xorg.conf (see above).

So there are some bugs left in the stable branch, but for every single of it, there’s a workaround. These bugs doesn’t hold me off building a media center PC, because I don’t even use a window manager If I set up a such with e.g. an Open Source Media Center application for Linux:My Media System.