i have the same problem and im not using the sp1 of vs2005 but when we remove the updatepanel all  intelissense and bug erros just disapears

this errors are just bugs they dont make ur aplication to stop work they just confuse u when something else isnt working u just need to find what isnt working and fix it then ur aplication will build and  will run just fine and for me if u delete the updatepanel everything just go back to normal and when u finish the page u just need to add the updatepanel again