I think the refs are doing a fair and honest job, but as the Old Firm generally get more attacking play in a match; there will be more opportunities for them to be awarded penalties/dangerous free kicks et al. With every Old Firm away game being televised live, it also allows replay after replay of dodgy decisions until the media and public can jump to their own conclusions.
It also doesn't help when the Old Firm players play act as often as they do, making refereeing even harder. See Lafferty v. Mulgrew/Aberdeen, Nakamura v. St Mirren, Lennon v. Juanjo/ICT, Lovenkrands v. Thomson/Celtic, McCulloch yesterday... the list goes on. It's up to the club to sort these clowns out, as they are making the referees difficult job even more impossible.
Out of interest, does anyone still have the video/GIF of Juanjo v. Lennon? I remember it being posted last year and I forgot to bookmark/save it.