In the Apple Forums a user pointed out that Leopard's version of the Anjal Tamil Input Method has at least two bugs: the output of the combination "tr" is wrong and the em-dash is not available. Luckily Leopard's Anjal system contains two text files which users can modify to fix such things or make other customizations -- Anjal.keylayout and AnjalTransliterator.txt. You will find them in System/Library/Input Methods/TamilIM/Contents/Resources (you need to do Control + click > show package contents on the TamilIM item to get to Contents).
On my iDisk you can find a folder called "New Anjal" which has modified copies of these files to fix the two bugs mentioned. Or if I haven't got it quite right, it should not be hard to do so yourself.