ifup wlan0 problem

[Eftaxiopoulos Dimitrios <eftaxiop@central.ntua.gr>]

I have an Intel 4965 WiFi in my Debian testing laptop. I can connect to the 
Internet via the sequence of commands

# ifconfig wlan0 up
# iwconfig wlan0 essid myessid
# iwconfig wlan0 key mykey
# iwconfig
# dhclient wlan0

which gives the following output in the terminal

Internet Systems Consortium DHCP Client V3.1.0
Copyright 2004-2007 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

wmaster0: unknown hardware address type 801
wmaster0: unknown hardware address type 801
Listening on LPF/wlan0/00:1d:e0:84:ca:e9
Sending on   LPF/wlan0/00:1d:e0:84:ca:e9
Sending on   Socket/fallback
DHCPREQUEST on wlan0 to 255.255.255.255 port 67
DHCPACK from 192.168.2.1
bound to 192.168.2.3 -- renewal in 936354735 seconds.

However, when I issue

#ifup wlan0

I get

Internet Systems Consortium DHCP Client V3.1.0
Copyright 2004-2007 Internet Systems Consortium.
All rights reserved.
For info, please visit http://www.isc.org/sw/dhcp/

wmaster0: unknown hardware address type 801
wmaster0: unknown hardware address type 801
Listening on LPF/wlan0/00:1d:e0:84:ca:e9
Sending on   LPF/wlan0/00:1d:e0:84:ca:e9
Sending on   Socket/fallback
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 3
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 7
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 8
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 7
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 13
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 18
DHCPDISCOVER on wlan0 to 255.255.255.255 port 67 interval 5
No DHCPOFFERS received.
No working leases in persistent database - sleeping.

and no connection is established. In my /etc/network/interfaces file, I have 
included the entries

iface wlan0 inet dhcp
        wireless-essid myessid
        wireless-key mykey

Any suggestion?
Dimitris