As it stands, the only way is to set up an "if xxx key pressed" for every letter.
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In the new version of Scratch that is about to be released (1.4) there is a new block 'ask' that will request user input and save it into a variable called 'answer'. See the post in Announcements: http://scratch.mit.edu/forums/viewtopic.php?id=17835 for more details about the new 1.4 release.
--Chris G.
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When will it be released?
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