Rails - May 2006 - Scaling the database for write intensive application

I am developing a large scale financial application which is going to be 
very intensive write-wise, and use a lot of transactions.

I can see how the the web and application servers can be scaled and load 
balanced quite easily, but not the database.

The setups I have looked at so far seem to deal with coping with a lot 
of reads, and minimal writes - this maybe because they are largely 
content sites not web applications as such. So they use one Master write 
database which is replicated  to memcached read-only databases and loads 
of static content caching.

I have looked at Multi Master Replicated setups but am worried about the 
lag time between replication. The integrity of the data is critical.

Is it a case of looking at database specific features or is there a 
generic solution. Has any one used XA style transactions on MySQL, 
because one big brick wall is that we can''t split the tables on to 
separate databases because transaction do not work over multiple 
databases. If we could do this I think our problem would be solved.

I look forward to your thoughts.

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