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 down
sudo ifconfig wifi0 down
svn -r 3876 checkout http://svn.madwifi.org/madwifi/trunk/ madwifi-ng
cd madwifi-ng
patch -Np1 -i ../madwifing-r3876.patch
sudo ./scripts/madwifi-unload
make
sudo make install
sudo depmod -ae
sudo 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