480 line displays let you input 480p digitally, even when sources were 480i.
720 line displays let you input 720p digitally, even when sources were 480i, 720p or 1080i.
Why shouldn't 1080p ?
HDDVD & BD will store 1080p on the disc and output it over HDMI (just not this month!)
DVD players now output 1080p over, and hidef will follow.
1080p displays internal processing of 1080i up to 1080p is usually deficient, that''s why you should have 1080p HDMI inputs.
Ignore that infamous, silly, mis-informed and grossly misleading article.