It is called Segoe UI, and comes bundled with Windows Vista, Windows 7, and Office 2007. As far as I can see, it contains the full complement of IPA symbols (excepting only the labiodental flap, U+2C71). Here’s what it looks like, with a screengrab of how Word 2007 displays some phonetic text in 10-pt size.
You will see that its small-cap i (the lax close front vowel) has the serifs I pleaded for last week (blog, 6 July); so does the ordinary upper-case I. The diacritics sit nicely in their proper places. The dental click symbol extends below the line, making it easily distinguishable from lower-case L. (The fourth line in my screenshot is a transcription of the Zulu phrase isicathulo nesigqoko ‘a shoe and a hat’.) I’m not really satisfied by the proportions of the implosive-g symbol — hook too big, bowl too small — but it’ll do.
Microsoft tells us
Segoe UI is an approachable, open, and friendly typeface, and as a result has better readability than Tahoma, Microsoft Sans Serif, and Arial. It has the characteristics of a humanist sans serif: the varying widths of its capitals (narrow E and S, for instance, compared with Helvetica, where the widths are more alike, fairly wide); the stress and letterforms of its lowercase; and its true italic (rather than an "oblique" or slanted roman, like many industrial-looking sans serifs). The typeface is meant to give the same visual effect on screen and in print. It was designed to be a humanist sans serif with no strong character or distracting quirkiness.Wikipedia says
Segoe UI is optimized for ClearType, which is on by default in Windows. With ClearType enabled, Segoe UI is an elegant, readable font. Without ClearType enabled, Segoe UI is only marginally acceptable. This factor determines when you should use Segoe UI.
It is distinguishable from its predecessor Tahoma and the Mac OS user interface font Lucida Grande by its rounder letters.
Segoe was designed by Steve Matteson during his employment at Agfa Monotype. Licensed to Microsoft for use as a branding typeface and user interface font, it was designed to be friendly and legible.
So it’s goodbye Tahoma, and hello and welcome Segoe UI.
Do Mac users have this font?
If all is well, and you have Segoe UI on your system, this paragraph should be in the font. aɪ ˈduː ˈhəʊp ju kŋ ˈriːd ɪʔ.
UPDATE: If all is well, and you have Segoe UI on your system, this paragraph should be in the font. Mac users should see Lucida Grande. aɪ ˈduː ˈhəʊp ju kŋ ˈriːd ɪʔ.