WxPerl is nice because I could make native GUIs for Mac OS X (pTk and Gtk-perl require X11).
And I believe perl/tk doesn't work at all on older versions of MacOS X, so choosing perl/tk means locking out users who haven't stayed on the $130/version upgrade treadmill. My perl/tk app was meant to be cross-platform, but I ended up deciding not to support it any more on any platform but Linux, because supporting Windows users was eating up large amounts of my time, and it was an open-source project that I wasn't making any money on. No matter how cross-platform one of these toolkits is supposed to be, I think making an app cross-platform still requires an order of magnitude more maintenance work once the app is mature. I've also soured on the whole idea of bringing OSS to the Windows-using masses -- I think it just sucks energy out of the OSS movement, and there's no real return on the investment, because those folks are happy with Windows, and are going to keep on using it.
I don't typically downvote; however, I disagree with the sentiment about bringing opensource to Windows. I'm not going to turn this into a flame war, so I'm going to try and be concise:
People who use FireFox/Thunderbird, Gaim, OpenOffice and VLC on Windows, will have a much easier time converting to Linux. By advocating these specific pieces of software I can make a move away from any one operating system much more seamless. I do believe in dropping support for archaic software, and for that I accept a downvote or two. On a modern site, targetting a modern audience, I would support a modern browser. I use CSS2.0 often, CSS1 & XHTML all the time, I only support IE 6.x and FF 1.x, and nothing more. (Though I expect my site to "work" for some degree on all recent browsers, because everything validates as strict.)
If this was C++, I would suggest looking into GLUT/OpenGL. GTK qualifies as 'portable-enough' for me.