I needed to know this and came here, before I remembered the Environment class.
In case anyone else had this issue, just use this: Environment.CurrentDirectory.
Example:
Dim dataDirectory As String = String.Format("{0}\Data\", Environment.CurrentDirectory)
When run from Visual Studio in debug mode yeilds:
C:\Development\solution folder\application folder\bin\debug
This is the exact behaviour I needed, and its simple and straightforward enough.