Strategy for dealing with large db tables

Question

I'm looking at building a Rails application which will have some pretty large tables with upwards of 500 million rows. To keep things snappy I'm currently looking into how a large table can be split to more manageable chunks. I see that as of MySQL 5.1 there is a partitioning option and that's a possible option but I don't like the way the column that determines the partitioning has to be part of the primary key on the table.

What I'd really like to do is split the table that a AR model writes to based upon the values written but as far as I am aware there is no way to do this - does anyone have any suggestions as to how I might implement this or any alternative strategies?

Thanks

Arfon