Solar Panels : Solar Panels Direct $1.74 per watt
Keep watching the website, their stock rapidly changes. Go in with a friend to beat their minimum orders - or craigslist/ebay off the extras; they have two or four piece minimums usually. Inspect panels completely on receipt, I have friends that were allowed to keep shipping damaged panels as well as getting free replacements.
If you purchase their house brand "Sun" line that use the dark, almost black colored cells you're purchasing the highest efficiency cells at 18% conversion - meaning more power per square foot. I own two of their 50w panels that exceed their label ratings by 15%!
A silicon foundry makes the dark (not light blue) PV cells for their own panels that are only available as professional install packages - sunelec is the only place I've seen selling them to the DIY crowd!!
If you are doing mobile panels 1) think cable locks to keep them from walking and 2) sand bags or gravel boxes to ballast the frames down as just one errant gust of wind can/will ruin the panel(s)...