My only consolation is that this fault is not intermittent.  :-(

Here's what happened.  I emerged a different (to my tried & tested rt2x00-9999 
wireless driver).  Then I uninstalled rt2x00-9999, but I couldn't get the new 
driver to work.  So, I unmerged it, remerged rt2x00-9999 (this is a CVS 
package) and I have not been able to make it work ever since.  I even 
recompiled the kernel in hope of getting it to work, but I can't get it to 
detect any Access Points:
=====================================================
# /etc/init.d/net.wlan0 restart
 * Starting wlan0
 *   Loading networking modules for wlan0
 *     modules: apipa arping ccwgroup macchanger macnet rename ifplugd 
iwconfig essidnet iptunnel iproute2 pppd system dhcpcd ip6to4
 *       ifplugd provides plug
 *       iwconfig provides wireless
 *       iproute2 provides interface
 *       pppd provides ppp
 *       dhcpcd provides dhcp
 *   Configuring wlan0 for MAC address 00:11:50:18:55:3F ...       [ ok ]
 *   ifplugd does not work on wireless interfaces
 *   Configuring wireless network for wlan0
 *   Scanning for access points
 *     no access points found
 *   Couldn't find any access points on wlan0
 *   Failed to configure wireless for wlan0                        [ !! ]
=====================================================

Here's some additional relevant info:
=====================================================
# iwlist wlan0 scanning
wlan0     Interface doesn't support scanning : Network is down

# iwconfig wlan0
wlan0     IEEE 802.11g  ESSID:""  
          Mode:Managed  Frequency:2.412 GHz  Access Point: Not-Associated   
          RTS thr:off   Fragment thr=2346 B   
          Encryption key:off
          Link Quality:0  Signal level:0  Noise level:0
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0
=====================================================

The device and its modules are loaded:
=====================================================
# lsusb
Bus 003 Device 001: ID 0000:0000  
Bus 002 Device 007: ID 050d:7050 Belkin Components F5D7050 ver 1000 WiFi

# lsmod | grep -i 2500
rt2500usb              31168  0 
80211                 157644  2 rt2500usb,rc80211_simple

# lsmod | grep -i  80211
ieee80211softmac       26880  0 
ieee80211_crypt_ccmp     6720  0 
ieee80211_crypt_wep     4032  0 
rc80211_simple          4416  0 
80211                 157644  2 rt2500usb,rc80211_simple
=====================================================

tcpdump doesn't show much when run immediately after I insert the adaptor:
=====================================================
# tcpdump -vv -i wlan0
tcpdump: WARNING: wlan0: no IPv4 address assigned
tcpdump: listening on wlan0, link-type EN10MB (Ethernet), capture size 68 
bytes
tcpdump: pcap_loop: recvfrom: Network is down
0 packets captured
0 packets received by filter
0 packets dropped by kernel
=====================================================

Would you perhaps know how I may be able to troubleshoot this thing before I 
lose my temper and throw it out of the window?

PS. I might have un/plugged it during probing/scanning - could that have 
caused some physical damage?
-- 
Regards,
Mick