Darren,
It's really hard to say off the cuff if you'll experience problems without examining your infrastructure and architecture of EV. If you have a simple installation, and a minimum of modules (ie, just mailbox archiving), then you should feel pretty confident about the upgrade. I do recommend that you upgrade to 6.0sp2.
I guess the only thing you have to really worry about is making sure that your archive and retrievals tasks get translated in to 6.0 tasks successfully (ie, check permissions, and do test syncs). Be sure to take note if you start seeing dcom and/or other errors pop up in the event log.
The best advice any of us can really give is to make sure you've got a complete backup of your environment. Just in case.
micah