Based on the Mauricio de Paulo's brand QGIS plugin called WKTRaster, I added raster support to the plugin. Likewise, the layer list widget for PyQGIS was slightly modified in order to get a basic table of contents displaying a number of layer properties.

It is a screenshot (Windows): 

As the installation process now includes an images folder, which is actually optional, I think it is recommended to show it again:

PREREQUISITES

Python, PyQt4, PyQGIS and libqt4-sql-psql.

How to install prerequisites in Windows: See this post.
How to install prerequisites in Ubuntu/Linux: For QGIS, see downloads official page. For the other programs use apt-get or Synaptic.

DOWNLOADS

For Ubuntu/Linux download both the postgis_viewer.py and postgis_viewer_imgs.zip files.

For Windows download postgis_viewer.py , postgis_viewer.bat and postgis_viewer_imgs.zip files.

HOW TO USE THE VIEWER FROM A COMMAND LINE

Ubuntu/Linux: Set the execution permission:

 $ sudo chmod 755 postgis_viewer.py 

 $ postgis_viewer.py -h localhost -U postgres -W secreto -s public -t pais -d geodb & 

 $ postgis_viewer.py -h localhost -U postgres -W secreto -s public -t rios -d geodb  

Windows:

Be sure you have the QGIS libraries enabled and run in a command line:

 $ c:\Python25\python.exe postgis_viewer.py -h localhost -U postgres -W secreto -s public -t pais -d geodb &  

INSTALLATION AS A pgAdmin PLUGIN (PostgreSQL 8.4)

Ubuntu/Linux:
Copy the postgis_viewer.py file into one of the folders of the environment variable PATH, for instance, /usr/bin/

 $ sudo cp postgis_viewer.py /usr/bin/  

 $ sudo chmod 755 /usr/bin/postgis_viewer.py  

Title=View PostGIS layer
Command=postgis_viewer.py -h $$HOSTNAME -p $$PORT -U $$USERNAME -W $$PASSWORD -d $$DATABASE -s $$SCHEMA -t $$OBJECTNAME
Description=View PostGIS layer
Platform=unix
ServerType=postgresql
Database=Yes
SetPassword=Yes 

Extract the postgis_viewer_imgs.zip file into /usr/bin (or the folder where you put postgis_viewer.py) and set execution permission:

  sudo unzip postgis_viewer_imgs.zip -d /usr/bin/ 
  sudo chmod 755 /usr/bin/postgis_viewer_imgs/ -R  

Windows:

Copy the postgis_viewer.py and postgis_viewer.bat files, and extract the postgis_viewer_imgs.zip file into C:/Program Files/PostgreSQL/8.4/bin/

Edit C:/Program Files/PostgreSQL/8.4/pgAdmin III/plugins.ini adding the following lines:

Title=View PostGIS layer
Command="$$PGBINDIR\postgis_viewer.bat" -h "$$HOSTNAME" -p "$$PORT" -U "$$USERNAME" -W "$$PASSWORD" -d "$$DATABASE" -s "$$SCHEMA" -t "$$OBJECTNAME"
Description=View PostGIS layer
KeyFile=$$PGBINDIR\postgis_viewer.bat
Platform=windows
ServerType=postgresql
Database=Yes
SetPassword=Yes

The postgis_viewer.bat file sets the OSGeo4W environment variables (Qt, Python, QGIS) to allow the execution of postgis_viewer.py The .bat file assumes OSGeo4W is installed in C:/ and PostgreSQL can be found in C:/Program Files/PostgreSQL/8.4/ You should check that file if you get some troubles while running the plugin.

NOTE:  The steps are based on PostgreSQL 8.4 and pgAdmin 1.10 has been tested on PostgreSQL <9.1 and pgAdmin <1.13.

KNOWN ISSUES:

• With PyQt4 < 2.6 you will probably get some weird display of the layer properties when dragging a layer. You may check the PyQt4 version from a Python command line this way (If you get a version less than 4.6, you have to upgrade your PyQt4 installation):

  >> from  PyQt4.QtCore import *
  >> VERSION_STR
  '4.6'  

• If you can not get the icons loaded in the plugin's window, check lines 62 and 65 of the postgis_viewer.py file. There you can edit the path to your postgis_viewer_imgs folder. Note that in Ubuntu/Linux you will probably need the full path whereas in Windows you just need a relative path (according to the tests done).
  

Finally, another screenshot (Ubuntu/Linux):

--------------------------------------------------------------------------------------------------------

UPDATE (2011/03/09): INSTALL THE PLUGIN ON PostgreSQL 9.0 (pgAdmin 1.12)

In Windwows, users need to be aware of paths for installing the plugin on PostgreSQL 9.0. The following changes need to be  done.

(Only WINDOWS)

• The folder to copy postgis_viewer.py, postgis_viewer.bat and extract postgis_viewer_imgs.zip is C:/Program Files/PostgreSQL/9.0/bin, however this may vary according to the installation of PostgreSQL 9.0.
• The plugins.ini file should be edited from C:/Program Files/PostgreSQL/9.0/pgAdmin III/plugins.ini
• The postgis_viewer.bat file should be edited changing the line that executes the postgis_viewer.py file. That line must be: python "C:/Program Files/PostgreSQL/9.0/bin\postgis_viewer.py" %*

 -------------------------------------------------------------------------------------------------------

REFERENCES

• Ivan Mincik's PostGIS Viewer: https://github.com/imincik/QgsPostGIS-Viewer
  
• Send and receive a signal between 2 Qt applications: http://www.qtcentre.org/threads/38453-Send-and-receive-a-signal-between-2-Qt-applications.?p=176716#post176716
• Single Application: http://www.qtcentre.org/wiki/index.php?title=SingleApplication
• Quantum GIS Development Team, 2011. Quantum GIS, Geographic Information System API Documentation. Open Source Geospatial Foundation Project. Electronic document: http://doc.qgis.org/