the DBA job greatly differs between the vendors, because the approach to make a RDBMS work is different
so the certs are Vendor-, Product- and Version specific
If you do an Oracle DBA cert, you will have to renew it at least for each major release (1g, 11g, 12x)
If you do a MS SQL cert, it's nearly useless for oracle products
And, my opinion, a serious DBA can't have more than one focus, I hardly find the time to keep up with Oracle's new features. To test them, know them, collect experience with them. Find Bugs, work around them. Really time consuming.