I tried to get aliasing by photographing a shirt which has a fine pattern and I did get a subtle pattern of colour shifts. I'm thinking perhaps I should try using live view to see if the LCD shows up a point where aliasing is bad. Luckily I don't do much people photography - out in nature, the resolution of the K5iis is awesome!
PS I just tried using live view - there does seem to be a point where colour aliasing is obvious in the LCD but it doesn't seem to translate to the final jpg image from the camera. I just tried opening the raw image and discovered that Paintshop Pro has no clue how to process it - the colours are completely wrong. I'll need to use the Pentax software to convert the RAW images from the K5iis to a format that PSP can use. The colour aliasing seems more obvious in the raw image. I'm still not concerned by this, because the shirt is the kind of thing I would expect to cause aliasing.