Copy paste the text shown below into a file called madwifing-r3876.patch.

--- madwifi/ath/if_ath.c  2008-11-07 01:05:07.000000000 -0500+++ madwifi-patched/ath/if_ath.c   2008-11-06 17:56:59.000000000 -0500@@ -3000,6 +3000,7 @@ ath_tx_startraw(struct net_device *dev, struct ath_buf *bf, struct sk_buff *skb) {        struct ath_softc *sc = dev->priv;+       struct ieee80211com *ic = &sc->sc_ic;        struct ath_hal *ah = sc->sc_ah;        struct ieee80211_phy_params *ph = &(SKB_CB(skb)->phy);        const HAL_RATE_TABLE *rt;@@ -3012,7 +3013,8 @@        struct ieee80211_frame *wh;

        wh = (struct ieee80211_frame *)skb->data;-       try0 = ph->try[0];+       //try0 = ph->try[0];+        try0 = (ic->ic_opmode == IEEE80211_M_MONITOR) ? 1 : ph->try[0];        rt = sc->sc_currates;        txrate = dot11_to_ratecode(sc, rt, ph->rate[0]);        power = ph->power > 60 ? 60 : ph->power;@@ -3036,7 +3038,8 @@        rt = sc->sc_currates;        KASSERT(rt != NULL, ("no rate table, mode %u", sc->sc_curmode));

-       if (IEEE80211_IS_MULTICAST(wh->i_addr1)) {+       //if (IEEE80211_IS_MULTICAST(wh->i_addr1)) {+        if (IEEE80211_IS_MULTICAST(wh->i_addr1) || ((ic->ic_opmode == IEEE80211_M_MONITOR) && (skb->data[1]&3) != 0x01) ) {          flags |= HAL_TXDESC_NOACK;    /* no ack on broad/multicast */          sc->sc_stats.ast_tx_noack++;          try0 = 1;

Check out the madwifi driver as follows:

sudo ifconfig ath0 downsudo ifconfig wifi0 downsvn -r 3876 checkout http://svn.madwifi.org/madwifi/trunk/ madwifi-ngcd madwifi-ngpatch -Np1 -i ../madwifing-r3876.patchsudo ./scripts/madwifi-unloadmakesudo make installsudo depmod -aesudo modprobe ath_pci

These instructions are similar to the ones written here (for a different revision):
http://www.aircrack-ng.org/doku.php?id=madwifi-ng