"Myriad (being a proper unicode font) simply doesn't have that checkmark glyph."
The Unicode character is 2610. Whether Myriad Pro really contains that character or not is an open question. "Windows 7 character map is misleading as it seems to display replacement glyphs from other fonts" Windows 7's Character Map doesn't always fill in missing characters from other fonts, there are lots of OpenType fonts where it shows U+2610 as missing, e.g. Verdana.
