I really like the acronym replacer plugin, but it has a few showstopping glitches. First, I like to replace all acronyms, but some (like AM--Ante Meridian) are the same as regular words ("I am Mark"). If the acronym replacer were case-sensitive, I think this issue could be avoided.
Second, and more seriously, it replaces acronyms inside HTML tags. For example
<img [...] title="XHTML Compliant" /> will be represented as
<img [...] title="XHTML Compliant" /> which is completely invalid.