I created http://www.websequencediagrams.com. PlantUML came several years later. I'm happy that my syntax is good enough to emulate in so many different projects.
I've been using PlantUML with Emacs's plantuml-mode quite successfully. I split the buffer into two, the top one containing PlantUML code and the bottom containing the rendered picture (yes, Emacs can view PNG files). Some snippets from my Emacs config may be useful for others: https://github.com/bdsatish/dot-emacs-24/blob/master/euchy/e...
I personally didn't know able plantuml, but I'm pretty sure you're correct about this one. Out of interest: Do you know if plantuml-syntax has a preferred extension?
If it isn't, the syntax is similar enough that this emacs mode is probably useful for PlantUML as well.
[1] http://plantuml.sourceforge.net/