Do u mean blocks having P2SH transactions can be validated by first Satoshi client, i.e. bitcoin-0.1.0?
P2SH was a soft fork so older versions which technically do not support it can still partially validate those transactions and blocks. However, in order to fully verify P2SH transactions, you would need to have bitcoin 0.5.4+
That means, in short, all bitcoin client releases were not backward compatible in nature. Does not it lead to the fact that Bitcoin has hard forked before?