Collected some interesting tutorials and articles for those wanting to explore hook_form_alter() and/or theming forms.

In no particular order:

HowTo: Theme a CCK input form
http://drupal.org/node/101092

Modifying Forms in Drupal 5 and 6
http://www.lullabot.com/articles/modifying-forms-5-and-6

Adding extra fields to the product (Ãœbercart)
http://www.ubercart.org/forum/support/3274/adding_extra_fields_product

hide cck labels in input forms
http://groups.drupal.org/node/5119

Drupal - How to Theme CCK Input forms
http://www.harostreetmedia.com/content/drupal-how-theme-cck-input-forms - same as the one above, but perhaps easier to read given the format.

Using hook_form_alter to fill in cck fields automatically and then hiding them from users (there’s no explanation here, just some code)
http://drupalbin.com/64

Module Weight - it appears that if you wish to affect the output of a form, you need to make sure that your module gets loaded after. This may help.
http://drupal.org/project/moduleweight

Forms API - How to use hook_form_alter
http://www.hiveminds.co.uk/node/3109

Drupal’s Form-handling System (for non-node data)
http://drupal.org/node/202756

Drupal 5: Making forms that display their own results
http://www.lullabot.com/articles/drupal_5_making_forms_that_display_thei...

Forms API Reference
http://api.drupal.org/api/file/developer/topics/forms_api_reference.html...

How to theme a Drupal form
http://agaricdesign.com/note/how-theme-drupal-form

HOWTO: Fully Theme And Customize The Drupal User Registration Form
http://www.chapterthreellc.com/blog/matt_cheney/howto_fully_theme_and_cu...

Using Theme Override Functions For Forms
http://drupal.org/node/112358

Move the help/description text
http://drupal.org/node/106144

Modifying your forms in drupal
http://www.molecularsciences.org/drupal/modifying_your_forms

This shows how to change a field from required to not required and some other stuff: http://www.ubercart.org/forum/support/2191/removing_sku_product_edit_pag...