Yeah, there really isn't any "rules" that one can apply in converting monsters from one edition to another - the way monsters (and characters, for that matter) are set up in 4th ed. is just too different.
So, it basically amounts to a series of gut checks. Personally, I would do the following:
1. Choose a general level for the monster (say, upper heroic, or mid paragon, etc.). This level will be based on the CR of the creature in 3.5, the level of similar creatures in 4th, and the needs of your campaign.
2. Decide which powers are most evocative of the creature. Choose 1-3 of them, 2-4 of them if elite, and 3-6 of them if a solo.
3. Decide what roles (controller, leader, lurker, brute, etc.) best fit the monster, based on the powers you have selected for it.
4. Flesh out the powers and convert them to the 4th ed. power format. Look at similar powers or similar creatures for guidance. Look at other creatures of similar level and roles to try to ensure that the damage and effects are appropriate.
5. Flesh out the rest of the monster.
In doing this, I can't recommend enough the D&D Monster builder - its a great tool and makes comparative analysis of monsters so much easier.
