Three comments:

1. I believe there is a bug in the code of \acopt:
Line 2
\ifthenelse{\boolean{acronym#1}}
should be
\ifthenelse{\boolean{acronym#2}}

2. One can avoid using \def, and stick to \newcommand (recommended when possible).
This can be done like this:

\newcommand{\acopt}[2][]{%%
\ifthenelse{\boolean{acronym#2}}%%
{\printlongacronym{#2#1}{\csname acronym#2\endcsname}%%
\setboolean{acronym#2}{false}}%%
{\printshortacronym{#2#1}}%%
}

\newcommand{\acnoopt}[1]{\acopt[]{#1}}

\makeatletter
\newcommand{\ac}{\@ifnextchar[\acopt\acnoopt}
\makeatother

3. The commands as introduced generate small space before the text that they print (I think only in the first instance, but I didn’t check thoroughly).
This can be avoided by placing comment sign (%) at the line endings inside the macro definitions.
(I commented each line’s ending, but this might not be necessary.)

Thanks for the code,
itay