I have searched and searched and not found a suitable replacement for Tomboy. Instead, I keep finding sites with other people looking for Tomboy on a Mac. The comments on the above blog entry reflect my thoughts pretty well.
Tomboy is a simple Gnomish note-taking program. When you press a hot-key, a new note pops up in a small text-editing window. If you close the window, the note is automatically saved. You can later retrieve the note by clicking on an icon on the bottom of your screen. A menu pops up with the ten most recently viewed notes, and if you want an older note, you can use a search field to find it. There are other things Tomboy does, but that is the essence and you simply cannot find it for the Mac.
Swikis are my fall-back tool. Even on Kubuntu, I used a Swiki for long-term notes that are more organized, saving Tomboy for quick things I wanted to jot down. Swikis are much more featureful, and their only real minus is that they use a web-based interface instead of using native windows and widgets. For example, on a Swiki, your text editing window is of a fixed size which is large for a little note yet small for a large note. Nonetheless, it is my note-taking tool of choice right now.
Sidenote is the best of the OS/X note-taking programs I tried. If you work only on your laptop, then it might well be a sufficient replacement. It has one fatal problem for me, though: as the name would suggest, the notes are all attached to the side of the screen. This might be workable on a 13-inch or 15-inch screen, but I do most of my work on a large external monitor. The notes are almost always far, far away from the main tool I am using. A version of Sidenote that used movable, resizable windows instead of being stuck to the side of the screen would probably be just what I am looking for.
Meanwhile, I am left with no good alternatives. I wish someone would make a Tomboy clone, either natively for OS/X or as something based on the Java platform. Judging from the traffic I see on the web, such a tool would have a lot of users.