Umm. 1.7ghz was a phtoshop job and is a prank/joke. NOT REAL.
It's also mentioned in the overclocking wiki.

I have tried 1.2ghz using old titan's kernel. It locked up my N900.
However, 1.1ghz was stable. So I can only assume 1.2ghz worked.

I haven't tried 1.2ghz with latest maemo25 kernel yet.