Drupal 7 template_preprocess_block I am trying to alter content of the system main block but only on the front page. I have added template_preprocess_block() in the template.php file, but is_front is FALSE even though I am on the front page of the site?

I use the Context module to control blocks, but the main block is the only one I have added using /admin/structure/block not Context.

Update
I have also tested hook_block_view_MODULE_DELTA_alter() but is_front is not passed to this function.

Update
Even though I disable the Main page content block front the Content region, I still see a node on the front page, publishing options set to published, promoted to front.
Doing this on a fresh install locally.

Enabling the Main page content block and setting "All pages except those listed" to  doesn't change anything - still looking at my node.
Use function drupal_is_front_page().