3G data up and running on my Nexus One using an O2 (UK) PAYG Sim.
Here's how to do it :
From the home screen...
press Menu hard key
select Settings
select Wireless and Networks
select Mobile Networks
select Access Point Names (APN)
Now you want to Add an APN...
press Menu hard key
select New APN
Now enter the following settings :
Name: O2 PAYG Data
APN: payandgo.o2.co.uk
Proxy: 193.113.200.195
Port: 8080
Username: payandgo
Password: password
Server: *LEAVE BLANK*
MMSC: *LEAVE BLANK*
MMS Proxy: *LEAVE BLANK*
MMS Port: *LEAVE BLANK*
MCC: 234
MNC: 10
APN Type: default
press Menu hard key
select Save
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