I just updated to 1.92 last night and I tried to registered my PSP on my PS3 but it doesn't seem to work with my custom firmware 3.52 M33-4. Earlier it does work with my Custom Firmware PSP without Register Device Option. Anyway, please test your PSP Remote Play with the official firmware, thanks!
Other new or revised features in version 1.92 (with 1.90):
[44.1/88.2/176.4 kHz] has been added as an option in [Audio CD Output Frequency] under [Music Settings].
[Type 2] has been added as an option in [Bitmapping] under [Music Settings].
[On] has been added as an option in [BD 1080p 24 Hz Output (HDMI)] under [BD/DVD Settings].
The [Image Quality Adjustment] option in the control panel has been changed to [AV Settings].
You can now play AVCHD-format video files that are saved on Memory Stick™ media or on the hard disk of a video camera.
You can now operate the (Slow (Back)) and (Frame Reverse) functions when playing video files saved on storage media or on the hard disk.
New for 1.92: Playability status with the PS3™ system has changed for some PlayStation®2 format titles. To check on the latest status, visit the search site for compatible titles (coming soon).
You can now adjust screen display settings while playing PlayStation® and PlayStation®2 format software.
You can now sort games that are saved on the hard disk.
[Add to Bookmarks] has been added as an option in the [File] menu of the Internet browser.
When an avatar is displayed during a voice / video chat, the avatar now moves in response to audio.
[Eject Disc] has been added in the options menu for discs.
You can now select [Group Content] when changing a file or folder icon.
You can update your system software by any of the following methods:
System Update »
Update using the PS3™ system's System Update feature.
Update using a PC »
Download update data using a PC and save on storage media* or a USB Mass Storage device. Copy the update data to the PS3™ system's hard disk and perform the update.
*An appropriate USB adaptor (not included) is required to use storage media with some models.
Update using disc media »
Update using update data that is included on a game disc.