UPDATE: 10th March 2009 - I've released an application to do all this for you automatically now, it only works on ROOTED G1's and it is availabe in the Android Marketplace as 'Latitude Enabler for Root' and also on GoogleCode latitude_launcher.apk
This has all got out of hand now, I became so frustrated with the Latitude situation on my UK RC9 T-Mobile G1 that I resorted to underhand techniques to re enable latitude on it after it disappeared again.
Google Maps is checking for a value in the gservices table, it checks that "maps_enable_friend_finder" has a value of "1". If it is then it shows the Latitude options in the Maps menu.
I added this value into the table on my rooted RC9 G1 and Latitude reappeared. I'll knock up an App today or tomorrow which resets this value and launches Google Maps, so that latitude is always enabled. If you want to do it for yourself now and you're braver than some and happy with words like adb and sql then you need to:
1) Connect your Rooted G1 to your PC via USB and run 'adb shell' (or use the G1 Terminal App no the phone)
3) cd /data/data/com.android.providers.settings/databases
4) sqlite3 settings.db
5) .dump gservices (to see all your current settings and check if maps_enable_friend_finder already exists)
6) INSERT INTO "gservices" (name,value) VALUES('maps_enable_friend_finder','1');
Now the Join Latitude button will be shown in Maps again, no need for a reboot.
If it doesn't appear make sure you have the values
<boolean name="FF_SHOWN" value="true" />
<boolean name="SHOW_MY_FRIENDS" value="true" />
in the file /data/data/com.google.android.apps.maps/shared_prefs/com.google.android.maps.MapsActivity.xml
hope it works and helps some of you out there. I'm off to write an app to do this automatically now.