WGS84 co-ordinates to shapefile points

[Biz King]

Hi All

I have a system (running beautifully and quickly on OS X.4 servers) with a complete GIS/GRASS
installation (courtesy of kyngchaos.com - thanks fo that).
I have a complete mapping system and planning system based on internal GIS data derived from
shape files, so all data is in meters and has relevant Extents values and is derived from shapefile
details, not WGS84 co-ordinates.

The problem I have is that I am being given GPS (WGS84) co-ordinates and need to convert them
for use within the same data sets.

Please could someone give me an idea of how to write a process that will handle this, or give me
some pointers (or tutorials) that will assist me to develop a solution for myself (I need to
understand this, not just to find a solution as I need to find a 'world' solution not just a 'local'
one.)

Some cut-down map file info is below plus a single coordinate that is similar to the ones I need to
translate.

Can anyone help me with this?
Thanks in advance.
cheers
Biz
----------------------------------------------
Lat/Lon details:
WGS84 Latitude: 1.234567
WGS84 Longtitude:  -8.234567

Mapfile details:
----------------------------------------------

MAP
 STATUS ON
# Extent -50000 -50000 655619  1220290
 Extent -150000 -150000 655619  1220290
 SIZE 900 900
 SHAPEPATH "/xxxxxxxxxxxx/osdata/export_wgs84/"
 FONTSET "/xxxxxxxxxxxx/book_src/fontset.txt"
 IMAGECOLOR 128 128 200
 WEB
  IMAGEPATH "/xxxxxxxxxxxx/osdata/tmp/"
  IMAGEURL "/xxxxxxxxxxxx/osdata/tmp/"
 END
   PROJECTION
     "init=epsg:4326"
   END

LAYER
  NAME "Pointers"
  EXTENT -50000 -50000 655619  1220290
  STATUS on
#  TRANSFORM FALSE
                 PROJECTION
                       "proj=latlong"
                 END

  TYPE point
  LABELITEM "NAME0"
  CLASS
                NAME "Pointers"
                STYLE
                     SYMBOL "PointerBlob"
                     SIZE 12
                     COLOR 255 128 0
                     OUTLINECOLOR 0 0 0
                END
                LABEL
                      TYPE truetype
                      FONT "eurostyle"
                      SIZE 9
                      OUTLINECOLOR 255 255 255
                      COLOR 0 0 0
                      POSITION auto
                 END
    END
END
END