I can hardly wait to see which tiara Kate will wear on her big, big day. However, there is a very long thread where every aspect of her potential wedding tiara is being discussed here:
http://www.theroyalforums.com/forums/f267/catherine-middleton-wedding-tiara-and-jewels-29273.html
But there will be one more tiara alert before The Big Wedding: the Emir of Quatar and his wife will pay a State Visit to Spain. The visit will start on April 25th and in the evening there will be a gala dinner.
I fear that tiara-wise it won't be very interesting. Sheika Mozah doesn't wear tiaras (but we may see some really big bling necklace on her). And the Spanish royal ladies hardly ever vary their tiaras anymore. So I guess we will once more see Queen Sophia with the floral tiara and Princess Letizia with the Prussian tiara.