How do I customise the starting number for orders, invoices etc in Magento 1.5?
There is actually a good extension for accomplish this task.
It allows you to customize the order ID in a lot of different way: for example you can use a combination of:
This is the ext. on Magento connect: http://www.magentocommerce.com/magento-connect/custom-order-id-8210.html
I personally tried it and it very good ( works fine with all my payment methods and I had no issue at all )