Just to reassure you all if you're worried about what you're looking at on the net being noticed at work.....
I do this as part of my job, so without giving too much away I'll say a bit about what I do (doesn't mean it's the same everywhere of course).
There are certain sites we don't allow at all - these are mostly things like sex, criminal activity, gambling.... Basically undesirable stuff, and the list of sites to block is bought in from a subscription service - but don't worry mumsnet wouldn't be on that type of list.
We could (but don't) also block other things which are defined as non-business related, but that's so hard to define accurately that we don't do it - however we can categorise (approximately) the type of sites people visit. I don't think mumsnet would fall into a category which would arouse any suspicion.
Another thing we don't do is to even attempt to say how long people have spent on the net - you just CAN'T know whether windows are being left open and other things being done at the same time, and our business means most people need to use it legitimately some of the time anyway. All we can actually do is say that pages were being requested regularly over a certain period, but if you're loading a long thread and spending several minutes reading it, it won't look a lot.
On most sites a web page will consist of many components - each picture being a seperate one for example). Because mumsnet (especially mumsnet chat) is largely text based, and importantly shows a whole thread at once rather than needing lots of clicks to read each message, it would come down as a fairly small number of net requests so it wouldn't look that significant on an activity check. Being mostly textual also tends to mean things like the talk/last day page isn't huge (although some of the long thread pages could be) so it wouldn't really register with what we do do, which is look at the amount of bandwidth people are using.
Occasionally if there is a question about what someone has been doing for any reason, or perhaps if they were under investigation for something, we might look in more detail at what they'd been doing, but in general unless it was so much that you were consistently showing up as one of the top few users in your company/dept (and remember mumsnet isn't that resource hungry) we'd probably never check on you specifically. There's just too much potential data there to actually look at all if it unless we have a real reason to.