The format hasn't changed between NS 6 and NS 7. The second argument to RunReportWithParameters is a string that uses name value pairs. To use the custom parameters you must know what those parameters are and put those in the name value pair separated with commas:
"param1=value, param2=value2, param3=value3"
Like the following:
parameterString = "From=2008-10-02T00:00:00, Name=%, Result Type=0, To=2008-10-15T00:00:00"
The names can be either the parameter or the parameter description. The same will apply with any report, it just requires knowledge of the reports parameters so they can be properly passed.
However, I believe there may have been a bug with this in earlier 7.0 versions of the ASDK which have only recently been corrected. I am attempting to verify this at this time. I will repost when I have verified.