I assume that those who would have voted for Allen in Virginia are the sort whose support will be solidified by evidence of racism, not alienated.
Quite true. I don't see how this could hurt him - it probably helps him outside DC suburbs. The only way this can have any negative effect if it somehow drives up the black turnout. Allen is still the heavy favorite in this race, barring further mishaps.
Now, as for the national nomination, he might be damaged goods now. Chances are, he isn't going to be a majority leader either. But a comfortably established senator - like, say, Trent Lott - he is likely to remain till the day he croacks.