Sounds good to me! Even not being too familiar with the API the above changes are much more readable and the consistency goes a longs ways towards making it easier to learn.

Since you said it could be done after 2.72 I have added it to the new UI Roadmap: http://wiki.blender.org/index.php/Dev:Doc/Projects/UI#Projects

Sorry that I didn't comment earlier. Was quite busy and tried to spend the time left with coding, rather than commenting ;).
The proposed changes seem good to me mostly, but I have a few more ideas to share:

Rename and reorganize button flags (but->flag)

• Rename states UI_ACTIVE and UI_SELECT to be more descriptive (see below)
• Add a new state UI_BUT_ACTIVE which can be used for radio buttons, checkboxes, tabs, etc.
• UI_BUT_DISABLED and UI_BUT_INACTIVE seem to be the same (or at least can be made the same), so we could delete UI_BUT_INACTIVE
• Same with UI_HIDDEN and UI_SCROLLED. They are more or less the same. UI_HIDDEN would be enough
• Split flags into but->flag and but->state. Both should be in UI_interface.h
• Rename flags to "UI_BUT_xxx" and states to "UI_BUT_STATE_xxx"

So this is how but->state could look like:

/* but->state */
enum {
    /* general states */
    UI_BUT_STATE_HOVER     /* was “UI_ACTIVE” */
    UI_BUT_STATE_PUSHED    /* was “UI_SELECT” */
    UI_BUT_STATE_ACTIVE

    /* more specific states */
    UI_BUT_STATE_TEXTINPUT
    UI_BUT_STATE_DISABLED
    UI_BUT_STATE_HIDDEN
    UI_BUT_STATE_ANIMATED
    UI_BUT_STATE_ANIMATED_KEY
    UI_BUT_STATE_DRIVEN
    UI_BUT_STATE_REDALERT
    UI_BUT_STATE_DRAG_MULTI
    UI_BUT_STATE_SCA_LINK_GREY
};

The rest can stay in but->flag.

Correct “wrongly” placed functions

Move functions to interface_utils.c
Afaik the purpose of “_utils.c” files is to collect functions that are useful in various places, providing something like a “function-toolset API”. Looking at the code, we have a lot of functions matching this scheme spread over all the editors/interface files. interface.c almost looks like a replacement for interface_utils.c, but interface_handlers.c has a lot of them, too.
Candidates for this are:

• ui_fontscale
• ui_get_but_val, ui_set_but_val, ui_set_but_hsv, …
• ui_get_but_string
• ui_convert_to_unit_alt_name
• ui_is_but_float, ui_is_but_bool, …
• ui_but_find_activated, ui_but_find_old, …
• ...

Other functions

• ui_but_menu (the right click menu) - from interface_handlers.c to interface_regions.c or interface_blocks.c (see below)
• ui_handler_panel - from interface_panel.c to interface_handlers.c
• uiDefAutoButR and uiDefAutoButsRNA - from interface_utils.c to interface.c
• ...

Rename button types

• OPTION and OPTIONN to CHECKBOX and CHECKBOXN (we should change this in the GUI, too – opinion from @Jonathan Williamson (carter2422) and @venomgfx?)
• Swap MENU and PULLDOWN - they've already been interchanged for years

Add moarz files

Add more files to /editors/interface making already existing ones cleaner and more compact and providing a better organization
E.g. interface_blocks.c or interface_buttons.c

• +1 for UI_BUT_STATE_*** changes.
• Id like to postpone moving functions for now (or do as second step, since it may involve header reorganization).
• 1+ for OPTION -> CHECKBOX
• not sure about swapping MENU/PULLDOWN. but this could indeed use some clarification.
• further organization I rather postpone a bit.