Honor points aren't decaying, but they are estimating
Lots of players have been having issues with the honor numbers after the last patch (and the recent decrease in honor gain), and the CMs are scrambling to get everyone OK and updated with the new honor points.
For the record, there is only one known bug: When your estimated honor hits 6554, the display (not the actual honor earned) will reset to zero. That means if you earn 10,000 honor in a day, your display will say 3,446 on estimated honor. It's a known bug, a fix is coming, and Drysc reminds players that the display has no effect on what's earned.
In that same thread, a few players question why estimated honor is shown at all-- if HKs can be calculated, why not show totals in real time? The reason is for diminishing returns: the server can calculate how many times you killed someone, but not who and when, so diminishing returns aren't showing up in estimated honor.
Finally, Neth says there are no current plans to reset honor for the Burning Crusade, so if you want to start stockpiling honor for the new coming items, feel free-- just remember that there is a max limit on honor earned, currently two times higher than the most expensive item. A few players are still worried that the CMs claiming there won't be a reset just proves that there will be one, but I'm siding with Neth this time: she says what she knows. Like the wind in the trees, who knows what tomorrow's Azeroth will bring?
