I agree, the only way I could think this would work would be if the robotx.txt file was on the root domain. I agree, check Webmaster tools, they will tell you under the sitemaps section about "Error: URL was blocked by robots.txt).
One thing to remember is that robots.txt is technically a suggestion to ask search engines not to crawl your site. They can choose to ignore it, though personally I don't know of any cases in which this happenned.