For iterating over an array, see http://www.jsviews.com/#fortag. (Lower in the page shows the use of the tag for looping over an array).
Also, some comments about your template example:
It looks like you are using a generated template, with the value of the columnName parameter inserted. So if columnName has the value "fooColumn" your template snippet is {{:#data['fooColumn']}} - which is actually equivalent to {{:#data.fooColumn}}, which can be further simplified to {{:fooColumn}}.
(I assume the value of columnName is a valid JavasScript name - not some value like "foo column", say, with white space - in which case your syntax would indeed be appropriate.)
Now, if #data.fooColumn is an array of objects each of which has a 'itemProperty' property, then using {{for}} to iterate over the array looks like this (slight change on David Ward's example):
<ul>
{{for fooColumn}}
<li>{{:itemProperty}}</li>
{{/for}}
</ul>
The generated form of that using columnName would be
"<ul>{{for " + columnName + "}}<li>{{:itemProperty}}</li>{{/for}}</ul>"