First of all, that you Stephen for your response. What I was looking for was a solution that didn't require me to copy/paste all of the "orphaned" notes (I had over 70) on my iPad/iPhone in order to get them copied to iCloud and thus available on my Mac. Here's a synopsis of what I did to solve this issue:
-- I purchased an app called DiskAid (http://www.digidna.net/products/diskaid) which allowed me to access the notes stored directly on both my iPad and iPhone and download them to my Mac as text files. However, I would still have to copy/paste to get these copied up to iCloud, so this wasn't a great solution.
-- It turns out that part of the problem was that the main Mac I was syncing both devices to was recently upgraded to Mountain Lion. In ML when you sync an iDevice via iTunes, the option to Sync Notes (check box under "Other" in the Info tab) is no longer available (ML has notes as a separate app, so they also dissappear from Apple Mail). However, my laptop was still running Lion, and this was the machine that I had been backing my devices up to until a couple of months ago, so I...
-------- Did a final backup to both devices on my iMac running ML by plugging into iTunes.
-------- Plugged each device into my laptop (still running Lion) and did a sync via iTunes after selecting the option to Sync Notes.
-------- Now when I looked at my Notes in Apple Mail under On My Mac, the iPad- and iPhone-only notes now appeared.
-------- This allowed my to simply drag and drop them to my iCloud account under Notes -- problem solved.
-------- Finally, I went back and synced my devices again with my iMac running ML by plugging into iTunes. This restored anything lost by the sync with my laptop running Lion.
I know the above won't work for everybody -- it is sort of unique to my situation.
Among the lessons to learn here is this:
If you have a Mac you are using to sync your iDevices to, before upgrading it to Mountain Lion, make sure that any Notes sitting in "On My iPhone" and "On My iPad" have been moved to iCloud or they can become orphaned.