The installation through the Solaris repository is as simple as typing:
$ pkg install mysql-50
to obtain MySQL 5.0.91 (status Jan 18, 2012) or click on this link to launch the interactive installer.
$ pkg install mysql-51
to obtain MySQL 5.1.37 (status Jan. 18, 2012) or click on this link to launch the interactive installer.
If you're attending the Oracle Open World 11, come hear me talking about pluggable authentication and developments around it (and if you're not attending here's one more good reason why you should :) ).