kjd, we've used Farecast in the past and have found it to be fairly accurate. the only downside to it (IMO) is that you can only look a relatively short time into the future, and we tend to book our travel more like 9 to 12 months out.
Regarding airfare for next summer, we just booked tickets for next May/June from Atlanta to Honolulu for $699 round trip. The fares were very variable - my husband is signed up for American Airlines' Farefinder and originally it was quoting us $1300 round trip (we need three tickets, so we were looking at nearly $4000 in airfare alone). Within two days he got an alert about the lower fare and we booked it right away. We're using mileage to upgrade to first class, so overall we feel like we got a pretty good deal. We're in the process of relocating from Los Angeles to Savannah, Georgia, and in the past we've paid around $600 to fly to Hawaii from LAX, so we were pretty excited to find reasonable prices from the east coast.