You cannot create a bank statement from paid invoices (customer or supplier) in v7.
You are supposed to create statements directly from unpaid invoices. The process is going to create the payment AND the statement line at once. This mechanism is supposed to be used on a more frequent basis than monthly if you want to follow up with payments during the month. You would then create several statements (eg. daily) throughout the month. This mechanism also allows you to take care of balancing, invoice reconciliation, statement reconciliation, and statement verification (based on starting/ending balances) all at once and with only one data entry.
The limitation of this is if you get monthly statements from your bank, then the documents in OpenERP do not reflect the reality of the paper documents in the sense that you will have many small statements instead of one big monthly statement.