It is tricky to change data formatted as text to any other format. Here's what worked for me in Excel 2003; I assume the same basic idea will work in Excel 2007.
I have in column A a bunch of email addresses formatted as text. Then in Cell B1, I type in a random email address, and Excel automatically formats it as a hyperlink. I copy this address all the way down column B (by highlighting column B, and choosing Edit->Fill.)
Next, I highlight column A, and copy the whole thing. I then highlight column B, right click, and choose Paste Special. From the screen that comes up, I click the radio button next to "Values and Number Formats". Now all the emails are formatted as hyperlinks!