SSD and PPC with Leopard 10.5.8
You can look here[1]. The members of this thread seem to agree that writing 1s (FF) to unused space by the OS allows the GC on the disk to do its job, effectively the same as TRIM. 

Unfortunately they devolve into insulting each others' knowledge of SSDs and Mac chipsets in general before doing anything useful. One of the commenters claimed there was a utility to write FF to unused blocks for PPC macs, but I've been unable to find it. 

[1]: [http://forums.whirlpool.net.au/archive/1953430]