From this (also here):

U.S. Customs and Border Protection Border Patrol agents said apprehensions of illegal immigrants have been on a decline in the El Centro Sector compared to last year due to an increase in Border Patrol specialty units.

[A spokesman said...] fiscal 2004 apprehensions were at 72,000 by the end of September.

"This year we have 51,000 apprehensions thus far," he said.

[The spokesman] said a number of different things could have contributed to the decline in apprehensions, including a shoring up of the sector's infrastructure...

Of course, it could also mean that they're just crossing in other sectors along the border.