First AOL did this, now Google is lowering google groups visibility. Seems to make sense since they don't seem very speedy about updating groups functionality.   Maybe they'll get wise and update it one day with a real conversation search.

Google got rid of the Froogle and Google Groups link on top of their search box, and introduced a link to Google Video instead. From what we know, only the most successful Google services get spotlighted on the homepage... in the past, services like Google Directory had to go.

Source: Google Video Added to Google Homepage