This is actually a thread I have been thinking of opening for some time.
As I see it, there are several interrelated aspects that are of importance here:
First and foremost, as an atheist I happen to believe that we have a general right to criticize religion in general and those aspects of it we deem to be harmful in particular. This includes - but is not exclusive to - their often extremely patriarchal nature.
In my view a particular difficulty with treating islam in this was is that a) there currently does appear to be quite a bit of negative sentiment about it within parts of society, that b) this cannot always be clearly distinguished from good old racism (albeit often poorly disguised as progressive values) and that c) (pseudo)feminist concerns are being cited as an issue of concern/abused as a fig leaf - often by those who otherwise couldn't care less about women. (Classical example: "We in the west respect our women!"). The tough question for me hence becomes how I can criticize religious principles while at the same time distancing myself from islamophobes and racists.
My own approach to this tends to be rather inconsistent in that I tend to argue to those advocating a certain practice that they are wrong while pointing out to some of their critics some of the more sexist bits of christian scripture. It's not something I recommend or favour, though.
One final factor that I find tends to get slightly lost in these kinds of debates is this: Roughly half of the believers in islam are women - and quite a few of them don't actually find their religion oppressive at all. Now, as a non-believer I disagree on that particular point - nonetheless, the idea of a bunch of non-muslims (extra points for white, educated middle class, non religious ones) looking to "explain" to muslimas how they are disadvantaged by their patriarchal believes is incredibly patronizing.
FWIW my DH is (nominally) muslim. As mentioned, I don't believe in any god, never mind religion. Every once in a while someone tells me - upon learning of his background - how "brave" I am for resisting the burqa. My paternal grandmother whose husband beat the living hell out of her until his cancer prevented him from doing so - and who 30 years on still maintains that husbands may indeed do this - worries that DH might not "respect" me. My new agey dad, who thinks that "souls" have an unchanging gender as which they keep on incarnating is worried that DH might hold sexist opinions.
Every time anyone says anything like this to me I feel like screaming into their face "stop patronizing me with your fucking prejudices, you moron!" I can only imagine what this must be like for someone who happens to believe that some of the stuff being criticized is actually objectively correct as per communication from the eternal big boss. I also happen to know for a fact that some muslimas do feel very negatively about such criticism.
I am wondering whether it is less patronizing for non-muslim feminists to express opposition or offense on behalf of ourselves rather than "them". I.e. is it preferable to say "as a woman I am offended to think that you do not seem to trust my judgment" rather than "It is offensive to muslim women that their testimony is not always regarded as equal to that of a man".
This approach probably has its own drawbacks, would love to hear how others see this, though.