They were not overclocked for sure. Those chips are "speed sorted OMAP2" and are really rated by TI as being able to run at 400MHz clock frequency. Have a look here:
http://www.mail-archive.com/maemo-de.../msg10619.html
https://s3.amazonaws.com/ppt-downloa...tablets565.pdf

Even today, there is a higher speed version of OMAP3530 (a chip which is similar to OMAP3430 used in N900), which can run stable at 720MHz instead of standard 600MHz. But this higher speed version of OMAP3 chips became available only recently, and I guess it was too late for N900: http://newscenter.ti.com/Blogs/newsr...ns-228688.aspx