On the Wikipedia entry for Roman numeral analysis, it says that major chords use the upper case numerals (e.g. IV for the F major chord in the C major scale) and lowercase letters for minor chords (e.g. vi for the A minor chord in the C major scale), but how about suspended chords? Since the 3rd is missing, the chord is no longer major or minor, so I'm wondering what is the convention used in this case?
Typically, in traditional classical music, non-harmonic tones like suspensions are not indicated in the Roman numeral analysis. You would simply notate the numeral and inversion for the chord to which you are resolving.
Here's an example:
In jazz and pop music, on the other hand, you may find the chord analyzed as IVsus or IVsus4, for instance. This is because in jazz and pop, notes that were traditionally considered to be non-harmonic in classical music are now considered to be part of the chord, and chord inversion is generally seen as less fundamental to the harmony.
Occassionally, you will see a conflation of figured bass and Roman numeral analysis that looks like this: http://www.sfcmtheory.com/harmony_supplements/chap_21.htm, but it is more typical with classical analysis to indicate non-harmonics independently of the Roman numeral analysis, as shown in the above example.
By the way, the wikipedia entry for Roman Numberal Analysis is somewhat inaccurate. I wouldn't rely on that as a source.