My product, a toolbar for Outlook Express, uses tooltips to display keyboard shortcuts. For some localization work, I needed to display an underlined character within a tooltip.
The only hint I could find was a reference to .Net tooltips.
Apparently, in both .Net and Win32 tooltips, a single ampersand will be stripped, a double ampersand will cause the next character to be underlined, and three ampersands will display a single ampersand.
Input Displayed &B B &&B B &&&B &B
Note that this will be affected if the TTS_NOPREFIX flag is set, which prevents ampersands from being stripped.