It says the client is associated at g-HT-20-2SS, which is 802.11n. Where does it say that it is only associated at 54 megs? Is the client far away from the access point?
type "show ap debug client-table ap-name <name of ap>" to see the TX rate and RX speeds of your clients:
(host) # show ap debug client-table ap-name AP-135
Client Table
------------
MAC ESSID BSSID Assoc_State HT_State AID PS_State UAPSD Tx_Pkts Rx_Pkts PS_Qlen Tx_Retries Tx_Rate Rx_Rate Last_ACK_SNR Last_Rx_SNR TX_Chains Tx_Timestamp Rx_Timestamp
--- ----- ----- ----------- -------- --- -------- ----- ------- ------- ------- ---------- ------- ------- ------------ ----------- --------- ------------ ------------
18:46:17:eb:a7:be ethersphere-wpa2 00:1a:1e:64:12:71 Associated 0x2 Power-save (0,0,0,0,N/A,0) 154 1277 0 4 58 65 31 32 2[0x3] Mon Feb 6 02:13:47 2012 Mon Feb 6 02:17:12 2012
10:93:e9:2a:d9:08 ethersphere-wpa2 00:1a:1e:64:12:71 Associated M 0x1 Awake (0,0,0,0,N/A,0) 7 6 0 0 39 6 0 36 2[0x3] Mon Feb 6 01:59:22 2012 Mon Feb 6 01:59:24 2012