The rules we've been using for our current group are as follows: Attributes that have not changed are converted over at the rate of Rating/2 (round down) + 1. A Strength of 4, for example, is converted to a 3. Quickness and Intelligence are multiplied by 1.5 (round up), and distributed among their new split-off attributes. So Quickness is multiplied by 1.5 and you distribute the points between Reaction and Agility, while Intelligence is multiplied by 1.5 and you distribute the points between Intuition and Logic. All conversions are done on NATURAL attributes, and augmentations are added on separately afterwards.
Skills that map to a Skill Group (like Stealth and Sorcery) are converted a Rating/2 (round down) +1. If you have specializations of these skills that can convert over to an existing skill (like Spellcasting or Sneaking -> Infiltration), you may opt to either fold it back into the skill (if possible) or simply convert it over to the Skill Group at Rating/2, and then have that particular specialization at 2 rating points above the Skill Group (this effectively breaks the Skill Group).
Skills that map over to a skill in SR3 convert directly, with a maximum skill of 6 (7 if your GM allows you to take an automatic Aptitude quality for that skill). In practice, we typically reduced the ratings of these skills by 1 point (any skill above rating 3 got this treatment), simply because it "felt" right. If you have multiple Specializations, choose a single Specialization to apply to that skill.
Computer and Electronics are tricky to convert over. We settled for a conversion of a Hacking Skill group equal to Computer (SR3)/2 (round down) + 1, along with Software and Computer (SR4) at the same rating. Electronics and Electronics B/R probably belong in the Electronic Warfare or Hardware skill respectively.
Since Vehicle skills were given such a big overhaul, we simply averaged the rating of existing skills in their respective groups. So a Car of 4 and Bike of 2 averaged out to a Pilot Ground Vehicle of 3. Alternatively, you can take the highest skill and subtract 2 from it, while adding a specialization for that particular skill. So a Car of 4 would convert to Pilot Ground Vehicle (Car) 2 (+2)
Knowledge Skill were all given the Rating/2 (round down) +1 treatment. Languages had their R/W version removed (no longer exists, except as a specialization), but ratings converted over directly, with the highest Language chosen as the "Native" language.
We also give each SR3 character a skill of 1 in both Dodge and Perception.
As far as ware, most Bioware can be converted to an Essence Cost equal to half the Body Index. Note that many pieces of Cyberware have an Essence reduction (Datajacks are only 0.1 Essence) and some simply don't exist anymore (Headware Memory is obsolete, no Math SPU, Encephalon is superceded by Cerebral Booster).
Most gear can be converted over into SR4 equivalents. Use your imagination, but it helps that most of the firearms are pretty well standardized (all Heavy Pistols are 5P, -1 AP, for example, except for the Ruger Superwarhawk).
Magic is also tricky, as Magic is generally much more expensive under SR4 than SR3. You may want to halve any Initiate Grade earned by the mage (round up), as well as any bonus Magic points from Initiation, prorating the Metamagics accordingly. This still makes SR3-converted Mages and Adepts more powerful than 90% of Mages and Adepts in SR4. Foci will have to be "broken down" into the new category-specific Foci in SR4, but Power and Weapon Foci remain pretty much the same as before.
Don't even think about converting Otaku. smile.gif