Heading south from the USA: Electronics/luxuries.
LED flashlights, digital cameras, nintendo DSs(big hit in Mexico), netbooks etc.
I ended up selling one of my cameras here, for the same as I paid 7 months ago, and I got to keep the best lens, there are simply no decent digital cameras here, SLR's seem to be non-existant, and fetch a nice price.
Doing it again, I'd have brought along a few entry level rebels, or whatever was cheapest on Amazon, a few sets of cards and kit lenses.
I have family members who load up every time they pass through the USA, and resell in Mexico where they live. A nintendo DS on sale they've got for about 200$, equivalent in Mexico is 400$.
I would love to learn about what to bring home, silver comes to mind, but I haven't passed any good places yet(though any tips would be great, I like having precious metals, and with it's current high value, it seems like a decent choice to drag back)