Based on my research here and on other forums, I believe that, as of July 7, 2006:
Firmware v2.70+ = NO Homebrew of any kind
GTA:LCS that has 2.6 update on the UMD = NO Homebrew of any kind
"IC1003" printed on circuit board under UMD slot cover + Firmware v2.01+= NO Downgrade possible
If I'm wrong, I would love to know what possibility I haven't explored.
Anyone? Anyone? Bueller?