I have attached a git patch against master that implements a new
parameter to clock tables, "tags".  This parameter is a tags-query as a
string and is used to filter the headlines which are consulted when
building the clock table.

In my search of the archives to see if this feature already existed, I
found a reference here:
  http://article.gmane.org/gmane.emacs.orgmode/17304
suggesting it was difficult.  The patch is not so large, though, so
perhaps I am missing something.

My rationale in implementing this feature was to keep track of the
occasional task item that is not billable, yet still makes sense to
include in the overall project structure.  Of course I could just avoid
clocking the task item, or manually delete clock lines before generating
a report, but this feature reduces the chance for error; no doubt there
are other workflows enabled with this feature as well.  I don't make
significant use of tags myself, but I know many do.

In order to maintain a sensible report, headlines that don't match the
tag filter may be included if they have descendants that do.  Any time
clocked directly on non-matching headlines, however, is excluded.

Specifying even a simple filter noticeably slows down clock table
generation for non-toy reports, particularly for clock table reports
with :step.  If there is no filter, though, there is no degradation in
performance.

Tag filter syntax is the standard one, as described at:
  http://orgmode.org/manual/Matching-tags-and-properties.html
Only tags are considered at the moment, although I suspect querying
against all properties would be possible (if even slower).

Examples:

* development
  CLOCK: => 1:00
*** task 1
    CLOCK: => 1:00
*** task 2                                              :must:
***** task 2a
      CLOCK: => 1:00
***** task 2b                                           :mustnot:
      CLOCK: => 1:00

Note I am using an unconventional but legal(ish) clock format for
brevity.  Clock tables are also pruned to only relevant lines.

[1] #+BEGIN: clocktable
|   | *Total time* | *4:00* |      |      |
|---+--------------+--------+------+------|
| 1 | development  | 4:00   |      |      |
| 2 | task 1       |        | 1:00 |      |
| 2 | task 2       |        | 2:00 |      |
| 3 | task 2a      |        |      | 1:00 |
| 3 | task 2b      |        |      | 1:00 |

[2] #+BEGIN: clocktable :tags "must"
|   | *Total time* | *2:00* |      |      |
|---+--------------+--------+------+------|
| 1 | development  | 2:00   |      |      |
| 2 | task 2       |        | 2:00 |      |
| 3 | task 2a      |        |      | 1:00 |
| 3 | task 2b      |        |      | 1:00 |

[3] #+BEGIN: clocktable :tags "-mustnot"
|   | *Total time* | *3:00* |      |      |
|---+--------------+--------+------+------|
| 1 | development  | 3:00   |      |      |
| 2 | task 1       |        | 1:00 |      |
| 2 | task 2       |        | 1:00 |      |
| 3 | task 2a      |        |      | 1:00 |

[4] #+BEGIN: clocktable :tags "must-mustnot"
|   | *Total time* | *1:00* |      |      |
|---+--------------+--------+------+------|
| 1 | development  | 1:00   |      |      |
| 2 | task 2       |        | 1:00 |      |
| 3 | task 2a      |        |      | 1:00 |

[5] #+BEGIN: clocktable :tags "must+mustnot"
|   | *Total time* | *1:00* |      |      |
|---+--------------+--------+------+------|
| 1 | development  | 1:00   |      |      |
| 2 | task 2       |        | 1:00 |      |
| 3 | task 2b      |        |      | 1:00 |

As you can see, in examples 2, 4, and 5, the time clocked on
"development" itself is being removed.  Example 2 illustrates the effect
of tag inheritance.

Adam