After a few weeks of banging my head against the keyboard – and only having small key impressions on my forehead to show for it – I think I’ve solved the problem of receiving the error:
“Keychain error
There was a problem saving to your keychain. Please try again or use Keychain Access to verify your keychain. Error: -2147415734.”
Essentially, my various Macs and iPhone were not syncing. Every time that I added a new date in iCal or a new entry in Address Book, I couldn’t be guaranteed that those would appear on the other machines. Further, using .mac/.me was like watching paint dry; it would either be slow, unresponsive, or sticky.
I’m noting because others might find this useful and, well, because I may forget in two days time how the problem was solved.
The issues seems to be around the Mac’s Keychain, which either becomes corrupted or experiences a permissions error. There is some information about it here, on Apple’s discussion board. The last post on the board was the most helpful, though not extremely so.
Here are the steps I took to remedy the situation:

1. Shut down all applications on all computers.
2. Ran Cocktail on all machines.
3. Opened Keychain Access (an application in the /Applications folder)
4. Opened Keychain Access Preferences and then clicked on “Reset My Keychain”
5. You get the standard warning but I said Ok
6. I restarted the computers, just for good times’ sake and re-entered my .Mac/.Me information BUT this time, I used my username only (e.g. username and NOT username@mac.com or username@me.com), as I believe part of the original problem stemmed from a multiple addressing issue on Apple’s part

I hope it’s helpful to someone out there.