When you export a file in LR one needs to nominate a colour space and a file type, TIFF, JPEG, etc.
Re-importing a file back into LR that has a different default colour space to the exported file may result in the unexpected changes in tone and perhaps other characteristics of the file.
The way to prevent these surprises is to use the import option and import files into LR 3 from file. Essentially all one is doing is making LR3 aware of files already on disk. No colourspace conversions occur and no surprises result.
Incidently, any files manipulatd in LR (using process 2003) will prompt a question from LR3 as to whether the file should be rendered using process 2010. This should not affect tonal manipulation but may affectadjustments for noise, sharpening and chromatic abberation.
Schewe or Digitaldog may have more to say on this subject.
My $0.02 worth