From what I can tell the <img> tag is the only inline-block by default. To be on the safe side I would recommend a class, you never know when changing all elements of a certain type will come back to bite you. Or, you could always make up your own tag and assign display:inline-block; to it. This way you aren’t changing the default functionality of standard elements…
EDIT
It also appears that button, textarea, input, and select elements are also inline-block
Sources:
According to this img is inline-block http://dev.w3.org/html5/markup/img.html#img-display
And here claims that button, textarea, etc. are as well: http://www.w3.org/TR/CSS2/sample.html
EDIT #2
While the source above claims that img tags are inline-block it seems (thanks to Alohci) that they are just inline http://jsfiddle.net/AQ2yp/
The following were tested in Firefox:
button is inline-block: http://jsfiddle.net/GLS4P/
textarea is inline: http://jsfiddle.net/235vc/
input is inline: http://jsfiddle.net/RFKe8/
select is inline-block: http://jsfiddle.net/5B4Gs/