Good morning all,
Thanks to everyone for contributing to this discussion.
First of all, some have wondered why we asked. Well, our advertising policy has always been informed by discussion with MNetters - obviously your views matter as the last thing we want to do is take a decision about the site that loads of Mnetters would object to. We are very conscious that you choose to come and post here and that there's no lock on the door 
so although clearly you can't please everyone all of the time we don't tend to take top down decisions on stuff like this. I strongly believe that without consultation, the site would be a very different (and less special) place.
Mumsnet's business model, however, is advertising. We need the ads to generate revenue to pay for our staff and servers. We don't think there's another model that works for mumsnet and so the ads are something we have to live with.
That said, we clearly won't work with everyone and we view our policy on advertisers and businesses we work with as a fluid one, determined in consultation with Mumsnetters.
In this particular case the advertising agency who works for McDonald's approached us to say they had a brief and would we be up for taking ads from McD's - they are looking to target parents with a new campaign they are doing. It doesn't mean we'll get their advertising if we say yes, just that we'll be one of the sites in the mix for consideration.
We have traditionally said no to McD's because we've had this discussion periodically and over the years quite a lot of folks have objected because they felt McD's buying policies, in particular, have been unethical (The Fast Food Nation critique). Last time we we had this discussion - maybe a 2 or 3 years ago - the reaction was around 50-50. We asked again this time because there is some suggestion that McD's are getting their act together on this issue a bit (although we don't claim to be experts). And indeed it seems that analysing the response on this thread (and the other one in AIBU) it does seem that the "don't minds" now outnumber the "do minds" by some way. So we are of a mind to change our policy on McD's
In general, we don't believe that display ads (ie the image and text ads around the place) on MN carry our endorsement. We wouldn't actively endorse a number of companies that we take display ads from - either because we'd never tried their products or didn't think they fitted over-well with the MN Brand. The only actual MN endorsements we give are as a result of our Product Reviews or following focus groups/testing. We will generally give closer consideration though to who we allow to be Discount partners, who we allow to sponsor bits of MN (and use words like sponsored by or in partnership with) and the type of language we'll use with relation to ads - we try to avoid the cheesy type of "we're so pleased to be partnering with xxxx".
We do also believe this is a site for grown-ups, not children. Much of the subject matter and language used is rather adult [understatement] 
and it's definitely an adult, not children's audience we are catering for.
Broadly speaking my personal view is that we should get the management of companies we end up boycotting, like Nestle, on for a webchat and put them and their policies under scrutiny. I tend to believe that engagement is the way to change things and that people should have a right to reply, but I do recognise that others feel that they'd rather not give such companies a platform. Anyway, thanks very much for your thoughts on this. As said we do operate a fluid policy and nothing is set in stone; and we are always very pleased to hear your thoughts and expertise on this or indeed anything else.