I was recently lucky enough to be delivering the SharePoint 2010 IGNITE ITPRO content in Redmond when a couple of the SharePoint CAT team dropped by for an informal chat with the attendees.
Thanks to Dan Winter
and Doron Bar-Caspi who spent over an hour taking on any question asked and really raised the bar on the day's delivery.
One thing that was mentioned during the session was an obscure STSADM command that works in both 2007 and 2010 but strangely enough has no 2010 powershell equivalent (yet ?)
STSADM -o localupgradestatus is documented here
and displays the farm and local server components that need to be upgraded. For all components, this operation can only check the local server, and cannot check any other servers in the farm. For example, this operation will not check the IIS server status on a different front-end Web server. As a result, you need to run this operation on all front-end Web server computers to get a full picture of the farm upgrade status. Databases in the farm are also checked even if they are on other servers.
The command has no switches and the output is an XML document with a structure similar to the following
 content database(s) encountered.
 content database(s) still need upgrade or cannot be upgraded.
 site collection(s) are contained in the content databases.
 site collection(s) still need upgrade.
 other objects encountered,  of them still need upgrade or cannot be upgraded.
So what does all this mean
The object node provides a description of the type of object being examined, its level which is indicative of the upgrade priority i.e. the upgrade order for this obejct and the status which indicates whether the item requires upgrading or not.
The final lines in the output give a status indication of the content databases, site collections and other upgradable objects.
All in all a very useful operation and one I would suggest is used along with preupgradecheck
when preparing to upgrade your farm to 2010.
The operation is also useful for SP2010 testing along with thetest-spcontentdatabase
powershell command for examining the upgraded farm.