Build providers are a "neat" way to automatically generate code during build in a website project. The problem is that R# 3.0 doesn't parse this generated code and it shows your code full of errors.
I personaly use SubSonic (ORM) which depends on build providers. R# 2.5 couldn't handle generated code (via build providers) either, but I had found a liitle trick. I disabled R# through Tools -> Add-in, then "Build" the website and then reenabled the R# plugin. This way R# 2.5 parsed the generated code, althought I had to do this everytime I opened a solution with SubSonic.
In R# 3.0 even that little trick doesn't work. Is build provider support scheduled for a future release? If not, will you consider a fix in the next nightly builds?