mIRC 6.3 custom-drew the titlebar when it contained unicode. This caused problems on Vista/7 as it caused the mIRC window to lose the Aero glass effect. As far as I know 7.1 no longer custom-draws the titlebar for this reason, calling Windows' Unicode APIs instead. The upshot of this seems to be that Windows' font-linking routines are now used instead of mIRC's.
On IRC you (and Mrs. Eenie) said you were running XP, which seems not to do much in the way of font-linking at all unless East Asian Language support is enabled in Control Panel -> Regional and Language options. I'm not sure what this setting changes as the fonts it installs don't seem to be enough, but once the setting is enabled the 7.1 titlebar will display correctly (as you confirmed). There might be a way to get the same results without needing to install 230mb of stuff, but I haven't been able to find it. An alternative is to change the titlebar font in Windows' display options to one that includes a wider range of Unicode characters.
