Wantasian - indeed as Icon 513 says, the Thai economy is not that linked to the US economy and has a dynamic of its own.
Your argument that since the USD is falling against the baht so prices of things should fall can only be true if most things in Thailand are denominated in USD or sourced from the USA. This obviously is not the case. Thai labour, land, food, cars, construction materials, clothing, medical supplies.....how much of these things do you imagine is supplied from the US?
From this
webpage you can see that the US provides only 6.3 percent of Thailand's imports behind Japan, China and Malaysia.
Prices rise in Thailand for many reasons, but overall, inflation is persistent. As you can see from
Xinhua News, the increase in Consumer Price Index for the year to June 2010 was 3.3 percent.
From this
webpage, you can see a chart of CPI increases for the last 30 months or so.
When comparing Sep with Nov hotel prices, you need to consider that High Season rates usually kick in around October.