Update #3: Yet Another UMD iSO Shrinker v0.1 (YAUiS) has been released by kenobi. According to the ReadMe file, "This program is all automatic: it scans the dirs, the files, and builds a new iSO of the smallest size possible all by itself."
Update #2: andychankc has let us know today that according to the DMU NFO File Rockman Dash: Hagane no Boukenshin (JAP) is the first PSP title that requires v1.52 firmware to start up... even with Hook Boot and Fastloader it will freeze at the loading screen. :stickpoke
Update: MikeG has now released UMD iSO Shrinker v1.0. This utility is a semi-generic variation of his previous PSP RipKits, and it currently supports ATV, NFS, Tiger Woods, Twisted Metal, and Untold Legends.
Today MikeG has released what appears to be the first of many PSP RipKits specifically designed for use with the previously-released PSP Fastloader v0.5. Those interested can grab the PSP RipKits for both ATV Offroad Fury (USA) and Need For Speed Underground Rivals (USA) (which incidentally goes from ~864MB to 259MB in size) now! Essentially what these PSP RipKits do is allow you to rebuild your own .iSO files of the titles with the unnecessary or "dummy" content removed, thus saving considerable space on your Memory Stick. Similar to PS2 RipKits, the process can be done manually (Tutorial HERE), however, for many users the PSP RipKits will make it a lot easier and save time. They are automated, and include both a PPF Patch and Win32 executable custom-made for use with the intended game to be rebuilt into a "Fastloader friendly" .iSO image file. Cheers to MikeG for his effort in making them, and post all feedback in our PSP Chat Forum.