A while back I had a person email me wanting some help getting their Samsung Galaxy S from Bell Canada working with their iPhone sim on the Telus network.  The problem was he could make phone calls but no data networks where showing up. 
 
   While I admit that my knowledge of those networks and the particulars of which phone works where is limited, I said I would help as much as I could.  I offered some advice, however he had figured it out after some help from the local Telus store employees. 
 
   Aside from a fluke timing issue where he tried it during the 2+ hours the Telus data network was down in his area, he also had to set the Access Point Name to "sp.telus.com".  Steps to set the APN:
1. After unlocking your phone (see link) and inserting the new sim
2. go through settings->wireless and network->mobile networks->Access Point Names
3. add an APN (Name= Telus APN= sp.telus.com)
 
I figured I would write about our experience (mainly his experience), so it could help others who may have the same issue.  
For more information about Access Point Names for Canadian carriers see this link at the XDA forums.