Ares 93: Little over a year and a half.

Hobbs: K bud, here we go.
First select the object you want to mirror. Move you camera so that its facing down, looking at its top view.
Go to tools and hit mirror, from there hit X AXIS and next hit COPY under clone selection, then ok.
Now you should see the object you wanted mirrored, already selected nest to your original object.
Here is where the fun happens. I use 3DS, so you may have to explore a bit in your program to find what i'm talking about.
Go to the top right hand corner and hit Utilities, then select both objects in the scene.
Back in the right hand side menu for Utilities, hit Collapse. Another menu will show up, and in that one select Mesh for OUTPUT TYPE and Single object for COLLAPSE TO
and then hit the Collapse Selected button, and bam , there you go. All the smoothing groups you had before will remain intact if you had done any.
You will need to convert in to edit poly mod and jest needed to merge the 2 halves.
From here go back to the Modify menu and select Vertex. Click and drag over the half you jest mirrored and move it to where you want it, normally on the X axis.
Zoom in and select the same vertx on both halves and hit collapse till the mesh is one solid mesh.
Then, hit your polygon button, and select your faces and smooth them. Then your done.
NOTE: Now let me say this, like i said i use 3DS MAX, so you may have to explore you program a bit to hind what i'm talking about. And i'm NOT one of the top modelers here, they may have a better way at doing this, but since i learned this my self i know it works.
I hope this helps.