While not all that rare, the six consecutive loses on all 3 exchanges hasn't happened since January 1991. As I post this, the markets are headed yet again towards another loss. While 7 days is somewhat rare, the longest losing streaks are 12 days on the DOW and S&P500 and an astounding 16 days on the NASDAQ. CNBC has a good summary of the 6, 7, and 8 day streaks for each market.
Edit: Make that 7 straight down days. Haven't ran the numbers, but it looks like record losses for that period.