From 1 April 2008, you will not be able to invest the first $20,000 in your Ordinary Account and first $20,000 in your Special Account. This restriction is in place because of the extra interest that you will earn in your CPF accounts from 1 January 2008.

However, you can continue to service your regular premium insurance policies (but NOT recurring single premium insurance policies or regular savings plans for unit trusts) and agent bank fees even if your Ordinary Account balance falls below $20,000. If you have already made investments using your Ordinary Account or Special Account balances, you will not be required to sell these investments.

Source: CPF Website