Obviously a reboot is required when loading a new kernel....

But in general, does yum leave some breadcrumb clue that some other process can check? For example, maybe there's something in the package files that yum sees and creates a "/rebootRequired" file which is always deleted upon booting?

If not, I think that'd be a cool idea.. the file could be
/rebootRequired.txt

and contain a plain text list of packages that caused the condition. :-)

Thanks