According to the PlayStation Twitter account, the 4.45 update includes trophy notification options and improves system stability - hmm.
We've reached out to Sony to clarify the situation. While the issues don't seem to be affecting all users, it might be wise to stay clear of the update for the time being.
Update: Sony Computer Entertainment sent us the following statement this morning:
"We are aware of reports that the recent PlayStation 3 system software update (version 4.45) has caused the XMB to not display on a small number of PS3 systems. We have temporarily taken 4.45 offline and are investigating the cause of the problem. We will announce when the system update is available for download as soon as possible. We apologise for the inconvenience."