What is a Flash Message - A Flash SMS appears directly on the phone's screen, instead of the 'Inbox'. Its an useful alternative to normal SMS when you want to catch the recipients attention immediately. The recipient has the option of 'Saving' the Flash SMS to his 'Inbox'.
Now, we can send Flash Messages in India only by using a service by Yahoo in which we have to send a special message which is costlier than a normal one. I guess the same is the scene in other countries.

But I just came to know about an amazing application for S60 phones called FlashSMS which lets you send Flash Messages from your phone without using an additional service. However, this application doesn't have the Phonebook support and Multi-Recipient feature. The author asks for a 'donation' and in return you get a Donation Code which enables these features in your application.
But in my opinion, even without these features this application simply rocks because it is not really crippled and works for unlimited period. All you need is to make an effort to type the recipient's number. So just download it and enjoy.