When you install Exchange sp2 on a SBS 2008 any issues you hit will not be because of SBS or unique to the SBS tool but stuff you would hit regardless of the installation and are pure Exchange issues.
For example… one that Nick Whittome hit during his testing was an error that he got while installing SP2:
“Setup previously failed while performing the action “Install”. You cannot resume setup by performing the action “BuildToBuildUpgrade”.
As you can see this is an issue that “normal” Exchange has seen. So if you hit issues, it’s not unique to SBS and you may need to google on the exact error message you get. Plan on about an hour for the download of the SP2 package (at least on my DSL speed) and about an hour to install SP2 on a box… and as I said before TAKE A BACKUP. Go into the Backup console and kick a special one.