I did a search on this, but did not find anything regarding it.
It is important to note that if you do go the route of downgrading (v1.51/v1.52/2.00) back to a v1.50, the only available v1.50 eboot you can use for this is the Japanese 1.50 update. This is of course because there was never a need to upgrade to v1.50 in the USA, but was needed for the v1.00 Japanese users. What does this mean? The v1.50 you use to downgrade is a Japanese version, essentially making your machine a region 2 device after the downgrade.
So, no USA UMD movie playback after the downgrade.
This does not bother me in the least, since I convert my existing DVD's to PMP format for playback anyway, but it is noteworthy...and I never read this in any of the downgrade tutorials I have encountered on the net.