There is nothing impractical about fencing off the southern border. There is no engineering obstactle in such a task that we cannot overcome. The only thing lacking is political will. Neither Bush nor the Democrats nor the four RINOs who voted yesterday for amnesty have any desire to control illegal immigration.
The American people overwhelmingly want illegal immigration stopped cold and legal immigration reduced. Unfortunately, our government increasingly listens only to elite opinion, and elite opinion is overwhelmingly in favor of mass immigration.
"There is nothing impractical about fencing off the southern border. There is no engineering obstactle in such a task that we cannot overcome." 
I agree. Well, if you don't count Florida's coast as a "Southern" border, anway.
You'll note that the link and quote up above shows that President Bush supports the 700 miles of new Southern border fence, by the way.
He pointed out, however, that fencing in all of our borders (think Hawaii, Alaska, Florida, Texas Coast, etc.) would be impractical.
But the 700 miles of new fence in the House bill is doable (and is supported by the GOP).