This problem has given me some severe headaches over the last few weeks, and I have found out that the pst files need to be kept as small as possible. To add to this little problem it appears that Outlook does not like the pst files to be resident on a server, and will hang if you have your data files pointed to a share on your server. This is a big shame, as most enterprises will not cater for files on local machines, especially files that need to be backed up (understandably so).
I have got around this by ensuring the affected users pst files are on the local machine, but this does give me the problem of ensuring these users have their pst files backed up in one way or another. I amended our shut down scripts to copy the pst files to the server when the user logs off so that they are backed up daily. But, out of the dozen or so staff who use Outlook, only 2 have had this problem, and why that should be the case I really have no idea, as they were all built from the same image, and they all use the same hardware/software configurations.
So, if anyone has any bright ideas on that, it would be great to hear them. Also, if anyone knows whether MS actually have this as an issue, as I have seen this problem posted on a number of blogs, and so far, no word from silicon valley