BTW, this is current "workaround":

<txp:article_custom file_id="_%" section='<txp:section />' >
  <txp:if_article_list>
    <h3><a href="<txp:permlink />" rel="bookmark" title="Read this
extract"><txp:title /></a></h3>
  <txp:else />
    <txp:if_article_id>
      <h3><txp:title /></h3>
    <txp:else />
      <h3><a href="<txp:permlink />" rel="bookmark" title="Read this
extract"><txp:title /></a></h3>
    </txp:if_article_id>
  </txp:if_article_list>
</txp:article_custom>

Let me deviate a little from the original issue.

This is "worst" that what I would have expected. The above workaround won't work, as
if_article_list inside article_custom always evaluates to true, even when on
individual_article context.
In other words, article_custom is considered a "list" context.
Try this snippet on your page template (change "a-section" value for a valid section
on your TXP install).

<txp:if_article_list>
this is a list
<txp:else />
this is an individual article
</txp:if_article_list>

<txp:article_custom section="a-section">
<txp:if_article_list>
this is a list // this always evaluates to list, even if in individual article context
<txp:else />
this is individual article // it seems that this place can never be reached
</txp:if_article_list>
</txp:article_custom>

I've tested the above snippet both on 4.0.8 stable and 4.2.0 RC (r3269) and got the
same output, so I'm still asking myself it this is a bug or just how things are
supposed to work.

Personally, I consider it a bug, as it doesn't seem to fit in the "standard" TXP way
of identifying contexts.
Yes, if you are "standing" inside an <txp:article_custom />, your context may be seen
as a list, as article_custom returns list of articles.
But then, a common usage for <txp:article_custom /> is to generate widgets that are
both used on both contexts (lists or individual articles). Inside those widgets you
may want to test if you are rendering it on a list or on an individual article.

Does it makes any sense?
Or I'm totally confused, or this is a bug, or things changed, or I need to sleep.
Sorry for my english.

Back to the initial issue, anyone may argue that using if_article_id outside an
individual_article context may not make any sense, and may be considered invalid. 

Using it inside an individual_article context, the question means: "is the article id
(the one for current URL) equal to X? (where "id" attribute value defaults to current
article if not specified)

Using it inside an article_list context, the question could mean: "on the current
URL, is it there any article_id value?" (which, of course, it should return false).

A few more thoughts on all this:

Regarding issue on #c2 (if_article_list being "true" inside article_custom, even on
individual article context), let me insist: it's a bug.

Regarding if_article_id, i spotted this scenarios on different scenarios

A) if_article_id on *individual_article* context:

A.1) <if_article_id> (no id attribute) is used _outside_ an <article_custom> tag =>
keep current behaviour: test currently being viewed article's id against default
attribute value. 
This case always return true, as the id attribute defaults to currently viewed
article id.

A.2) <if_article_id id="X"> is used _outside_ an <article_custom> tag => keep current
behaviour (test currently being viewed article's ID against "X"). 
May return true or false.

<my brain may be reaching a dead-end street below here>

A.3) <if_article_id> (no id attribute) used _inside_ an <article_custom> tag => keep
current behaviour: test article's id of current <article_custom> loop against a
default value (which equals the id of the article currently being viewed). 
May return true or false.


A.4) <if_article_id id="X"> is used _inside_ an <article_custom> tag => keep current
behaviour: test article's id of current <article_custom> loop against "X". 
May return true or false.

(being that every point above says "keep current behaviour", I'm not yet sure where
I'm heading to nor what's my point :P)

B) if_article_id on *article_list* context:

B.1) <if_article_id> (no id attribute) is used _outside_ an <article_custom> tag =>
new feature: return false.

B.2) <if_article_id id="X"> is used _outside_ an <article_custom> tag => new feature:
return false.

B.3) <if_article_id> (no id attribute) used _inside_ an <article_custom> tag => new
behaviour: return false
See original post on this thread: currently it doesn't return nor true nor false, and
workaround on #c2 is not currently possible.

B.4) <if_article_id id="X"> is used _inside_ an <article_custom> tag => keep current
behaviour: test article's id of current <article_custom> loop against "X". 
May return true or false.


Ok, I'll stop by now, sorry if this is too much non-sense.

Some ongoing discussion related to commend #2 here:
http://forum.textpattern.com/viewtopic.php?id=32216