There is something close - PasswordWallet is shareware for the Mac and Palm, but it doesn't use the Mac's native Keychain application. So what?
To get/install Secret!, web to the Linkesoft website and download the secret.zip file. Unzipping it produces 3 files worth bothering about:
<as root>
ln -s /dev/ttyS0 /dev/pilot
chmod 666 /dev/ttyS0
<as siemsen>
pilot-xfer --port=/dev/pilot --install Secret\!.prc
then push the HotSync button on the cradle
To install the desktop into Windows, open Internet Explorer,
go to the Linkesoft website and
download the application.
Now just enter data into the application on the Palm.
To use the application with its Windows desktop version, you have to understand the relationship between Palm "categories" and desktop "records". I don't :-) But I did learn that after writing new records in the desktop application, you copy them to the Palm by pressing the HotSync button on the Palm cradle. Similarly, to copy new records from the Palm to the desktop, you do the same thing.
There is no Secret Desktop application for the Mac, but there is a freeware thing named Secret Viewer. It lets you see the contents of the backed-up Secret database on your Mac.
I keep my Secret database on my primary Palm. To back it up to the secondary Palm, you can't just HotSync the secondary Palm. Jeff and I couldn't do it without doing a sync on the primary Palm and then doing a full Restore of everything onto the secondary Palm. To do a full Restore, start HotSync Manager, the pull-down HetSync->Conduit Settings, then select Backup, then click Conduit Settings and check the Restore button, then hit the HotSync button on the cradle.