I worked with ProCenter, across from Immigration and a bit further down the soi (full adress can be found in Marc's life-saving listings, just type rental in the search box).
I rented a Canon 5D, 24-70 f/2.8, 16-35 f/2.8, 70-200 f/2.8, 580ex flash, memory cards, etc. It cost me 30,000 baht for one week (5 days). I noticed day rates were roughly a 1/50th of the retail price at Fotofile. So if something costs 50,000 baht, it will be available for 1,000/day. Deposit for all of this was 180,000 baht which is not that much since the replacement value for this gear would be more like 250,000 baht or something. The gear had seen lots of use but worked fine. If your credit card can take the deposit amount, use that, otherwise he takes cash, which is what I did. He has a big safe

Of course, he has Nikon as well, and medium format, and just about anything really. Just call him, the number is listed here.
I'm very happy I rented because it taught me that while an f/2.8 zoom may sound like the Rolls Royce of lenses, I can't take the weight (not to mention the price). It also allowed me to witness the amazing 1600 ISO and sharpness of the 5D for myself and in real-life situations. In the end, I bought this (a couple days ago from Fotofile): Canon 5D, 24-105 f/4 IS, 17-40 f/4, 50mm f/1.4, 580ex flash, 430ex flash, STE2 controller. Just to say renting can help even if you plan on buying later.