From Howard Katz (HKatz@FocusedManagement.com):
Simply code your text field as "Allow multi-values", then on the Options tab, check "New line" for "Separate values when user enters" and "Display separate values with". This plain text field will then be displayed as a TEXTAREA. You can set the HTML attributes of the field to adjust the size of the TEXTAREA.
From Justin Freeman:
Note that you'll need separate web and Notes forms to do this: Add some passthru HTML text where you want the textarea to show up:
Then create a hidden computed field at the bottom of the page. Set the value to "comments" (the same name as the field).