Rumors are running rampant about the possibility of the 1.50 to 1.00 Firmware downgrader and PSP Updates has some information regarding it.
Please also know that there have been some reports of users bricking their PSPs while using this downgrade method. Hackers are speculating that only virgin 1.50 PSPs (PSPs that have never been upgraded/downgraded) will be able to successfully downgrade from 1.50 to 1.00. To put it another way; If your PSP was bought in the store as a 1.50 and has never been upgraded to 2.00 and then downgraded back to 1.50, the downgrader is reported to work. Of course all of this is still speculation, so please approach this with caution! We will keep you posted immediately as more information surfaces.
I personally do not recommend even attempting this as the possibility of bricking your PSP is fairly high, but it is your PSP after all, I just report the news.