What is the best way to implement input field and button behavior as displayed here:

What is the best way to implement input field and button behavior as displayed
Share
Sign Up to our social questions and Answers Engine to ask questions, answer people’s questions, and connect with other people.
Login to our social questions & Answers Engine to ask questions answer people’s questions & connect with other people.
Lost your password? Please enter your email address. You will receive a link and will create a new password via email.
Please briefly explain why you feel this question should be reported.
Please briefly explain why you feel this answer should be reported.
Please briefly explain why you feel this user should be reported.
I like the answer of ScottS, but just to have an alternative: you could use table-like behaviour in CSS:
CSS
HTML
http://jsfiddle.net/willemvb/VaFSP/