I've been looking into this as I've noticed the same and I've also noticed that, although you can select
Bitstream under the
BD / DVD Audio Output Format (HDMI) setting, this doesn't affect the behaviour for games where it still decodes Dolby Digital onboard to PCM. There's only two solutions to this that I can see (NB - this is for an older PS3, not the new slim model, I've no idea if it's the same or not on the slim):
1. Go into
Sound Settings, then
Audio Output Settings, select
HDMI, then select
Manual. Leave
Dolby Digital 5.1 Ch and
DTS 5.1 Ch ticked and then scroll down the list and untick all the
Linear PCM options which aren't greyed out. You'll find as you untick one, other
Linear PCM options above become available to untick so untick them as they become available. Eventually, you should be left with only four ticked at the top, the Dolby Digital and DTS settings and two
Linear PCM 2 Ch. settings which you can't untick as they are greyed out. If you scroll right to see the confirmation list, it should look like this:
Confirm this and now, since the PS3 doesn't think it can output Linear PCM, it will send Dolby Digital or DTS natively over the HDMI for everything. Of course the big issue with this is if you use your PS3 for Blu-Rays since the PS3 can no longer convert to PCM, so you won't get HD audio, just the regular Dolby Digital / DTS core tracks. This means using this option, you'd have to constantly turn these settings off and on.
2. The other method would be to connect an optical lead from your PS3 to the amp, as well as the HDMI lead. Since the optical lead can't handle 5.1 PCM, it won't try and send it. So when playing a game, switch to the optical output through
Sound Settings, then
Audio Output Settings, select
Optical Digital and then tick the boxes for
Dolby Digital 5.1 Ch and
DTS 5.1 Ch if they're unticked. You may also need to set your amp to use the optical input for sound rather than HDMI. This will mean Dolby Digital and DTS are bitstreamed across the optical. When you want to watch a Blu-Ray with HD audio, you'll then need to reverse this by going into
Sound Settings, then
Audio Output Settings, select
HDMI and then
Automatic. Again, change your amp to use HDMI for sound instead of optical and HD audio tracks will be decoded to PCM.
At any rate, if you use the PS3 for both games and Blu-Ray, either method is a bit of a pain and I would be tempted to just let your PS3 decode the Dolby Digital to 5.1 PCM...
EDIT - Ha - just seen you have the slim model - typical! Well, give it a go on the slim and see if it works, would be interesting to know. Alternatively, maybe you get different settings on the slim, such as the option to tick Dolby TrueHD and DTS HD Master Audio during step 1. If so, this could well sort it so you don't need to keep changing between the two...