Hi Hawk,
For joining two pre-existing objects:
Try a "Vertex -> Snap Together by Distance" command with the vertices to be joined selected. I use a tolerance of 0.0005, which is probably fine for most cases, but it depends on the size of your geometry what tolerance you wish to use. Once this is done, "Object -> Merge" them and perform an "Object -> Optimize Vertices". This should get rid of your seams.
For mirroring:
As luuckyy suggested, when mirroring, make sure the vertices at the "seam" where the mirror will occur are exactly lined up. It's best to move your object so these vertices are exactly on the axis where the mirror will take place. For example, if mirroring along the X axis, position your model so that the "seam" vertices are exactly on the X axis before hitting the switch. To be doubly sure that they are on the axis, use the "Vertex -> Align to Axis" command on these vertices prior to mirroring.
When you are using "snap to grid" you would normally expect that all vertices were in the exact same spot, but, unfortunately, this still isn't necessarily so.
The reason behind this is often due to the way computers perform floating point arithmetic. It's a fairly involved explanation, but suffice to say, you may have two points that you think are at 0.0 on a given axis, but one of them may be off by a *very* small amount.
There's actually a Microsoft knowledge base article that gives a light explanation of this issue at
http://support.microsoft.com/default...b;en-us;214118. The article applies to Microsoft products, but the principle applies here as well.
Good luck,
Dennis