I have a Humax PVR 9150T set up as in section 2.3 of the manual with a Playstation 3 instead of the VCR. So the setup is:
PS3 -> into the VCR SCART slot on the Humax -> SCART from TV slot on Humax to -> TV
(see linked pic here: http://img820.imageshack.us/i/humax.png/)

My problem is I can only see the output from the PS3 when the Humax box is off (in standby) and I have to disable the "Power Saving in Standby" feature.

If both the Humax and the PS3 are on and I press the source button nothing happens. If I have the Humax box on, the PS3 off and then turn on the PS3 - it doesn't change the picture. If I don't disable the power save in standby feature then when I turn off the Humax with the PS3 on all I see is a quick flash of the PS3 screen before it goes blank. This used to work fine with the DVD player (which was also connected this way) which the PS3 has replaced. I could just turn on the DVD player and provided the Humax box was on, it would flip over to show the DVD.

It may be a problem with the PS3 not sending a correct signal on the SCART but it is still annoying. Is there any workaround for this?

I've tried changing all of the display settings on both the Humax and the PS3 with no luck. Please help!

You would need to have power-saving set to off, and have the Humax in standby to use the PS3 (as it sounds like you've already discovered). I'm not familiar with the PS3, but on the PS2 the SCART connection was just via a 3x phono adapter, so there was no switching signal, maybe the PS3 is the same? Your DVD player would've almost certainly provided a switching signal. You'd have to disable the standby power saving in order to power the circuitry that provides the SCART passthrough.

Ok. Well thanks for answering. I figured as much but thought it was worth double checking.

Don't take my answer as gospel truth, I was only speculating. I don't actually own a 9150.

As far as I can see the PS3 does not have any equivalent to a SCART connection. With a full SCART to SCART connection there are additional signals carried alongside the audio and video.

One of these is a switching signal on pin 8 of the SCART. When you turn on a DVD player for example a voltage is applied to pin 8. The device on the other end of the SCART lead detects this voltage and switches over to the SCART socket the voltage appears on.

That is how the 9150 switched the SCART feed to the TV between itself and the DVD player on the VCR SCART. However the PS3 doesn't produce this signal so the 9150 doesn't know it's turned on and so won't switch over.

There is a button called SOURCE concealed under the slide down flap on the remote which might switch between Humax and PS3. I don't have anything on my 9150's VCR SCART so have never used it (and it's 5 miles away at the moment so can't try it anyway )