With the somewhat disappointing announcement regarding email not being available natively on the BlackBerry PlayBook at launch, we knew it was just a matter of time before apps starting springing up to deal with the problem. The first I've seen is called Early Bird, by Pepper.pk.
Written specifically for the BlackBerry QNX OS, Early Bird runs locally on your PlayBook so you can store, read, and respond to email even when you don't have an internet connection. POP and SMTP protocols are supported, so you will have access to popular email clients like Gmail, Yahoo Mail!, Microsoft Exchange via POP and most ISP provided email accounts. No word yet on pricing or when Early Bird will be available, but it's one that I am definitely interesting in downloading as soon as I can.