Same as Poledra - I either get nothing at all (but the rest of the www is working fine from the BlackBerry) and eventually a timeout message, OR a message that the webpage is unreachable. I once had a message "there is insufficient network coverage" but other websites are no problem.
This is with all MN addresses, including the landing page at www.mumsnet.com so is not necessarily related to any bookmarks or mobile pathways.
Usually, I get - nothing! The page starts loading but gets nowhere. I tried leaving it for over 5 minutes (yes I timed it) and it was still trying to load. Sometimes I get a webpage unavailable message, or a connection timeout.
For the last two days - whatever web address I use (including just www.mumsnet.com) - I cannot get access to MN using my BlackBerry. I am currently using my iPad and all is well using a PC. But during the day I am only able to use the BlackBerry. I am getting a variety of messages, but usually "web page not available".