If you're using iPad or iPod touch, you cannot send a text message to a non-iOS 5 device. BUT, with your iPhone you can send it as SMS (of course), but you may use iMessage to text the iPad user, and the iPad user can iMessage your iPhone.
Just think of it as, if it's not a phone with a carrier, you cannot use iMessage to communicate with phones that are not running iOS 5.
Example: Steve and Frank are using iMessage to communicate. Steve uses his iOS 5-equipped iPad to send an iMessage to Frank's iPhone 4 (also equipped with iOS 5), with full functionality of features. Lucy is using a Blackberry Bold, Steve tries to send an iMessage to Lucy's Blackberry from his iPad, but iMessage cannot find her because her device is not an iOS 5 device, and therefore is unable to text her.
Frank now wants to send Lucy a text message from his iPhone 4, so he does and it sends as a standard SMS text message, not with all the features of iMessage.
Lucy gets a brand-new iPod touch, fully equipped with iOS 5. Now Steve can reach her with iMessage on his iPad. Frank can also iMessage Lucy's iPod touch from his iPhone 4 without having to send it as SMS. The three of them laugh and play, living happily ever after.