1. Whether or not the dealer will actually do much cutting with the saw will vary (they may not have any to cut...) but definitely the saw should be ready to go, and the dealer should be able to show you basic operation and maintainance type stuff. (He should also give you the manual, which it won't hurt to read...)
2. In the manual you will see a minimum specification for your oil. If what you are running in your bikes meets that spec, go ahead and use the bike mix, otherwise get the saw oil and use that. I run brand name saw oil and the highest grade brand name pump gas (i.e. Exxon, Mobil, Shell, etc - no generics) I can get. No real benefit after 93 octane or so.
3. "Safety chain" has been compared to training wheels for chainsaws. It has a design that is supposed to reduce the chances of kickback over pro-chain, but don't put that much faith in it.... Many feel that it greatly reduces cutting performance. Since the 5100 is a "pro-saw" as opposed to a consumer grade, it may come with pro-chain rather than safety chain, which I feel is a better choice, as I found it frustrating to cut with the safety chain. You will also want to learn how to maintain a chain - get a proper size chain file, a raker file, a raker guage, and some file handles, and / or one of the various filing guides - and get the dealer to show you how to use it. IMHO the only time you should grind a chain is if you hit a rock or peice of metal, and need to do MAJOR restoration on it.
4. When negotiating for the saw, I'd go for at least one extra chain, possibly more, and an extra bar - not because you are likely to burn it, but because if you get stuck, and only have one saw it is good to be able to unbolt the powerhead and put on a new bar and chain. (I'd consider making the extra bar as long as the saw is rated for - that way you also have a long bar for big wood...) Also try for good deals on safety gear - helmet/muff/mask combo, chaps, chainsaw boots (or at least steel toes). (BTW, I wouldn't count on your MX gear offering much protection) Filters are mostly cleanable, but it wouldn't hurt to have a spare.
5. Chainsaws don't need a lot of break-in, the usual advice is to fire it up and put it to the wood - keep an eye on it though as it will run differently as it loosens up and wears in - typically around 15-20 hours it will benefit from fine tuning on the carb. (If you are trying for dealer independence you might want to get a hand held electronic tach - useful for carb adjusting)
6. There aren't all that many brand specific parts that you'll need - and you can get them online if need be, I know Amicks carries a great selection, and will ship overnight if needed.
7. If you can find a training course on saw use and safety in your area, it is a good investment.
8. If you are going to be doing a lot of tree felling or even just log cutting, there are a few non-saw accessories that I've found useful - a pair of pulp-hooks (get the ones w/ replaceable tips), a felling lever, and possibly some plastic wedges (though that's a little advanced)
Good luck, and always think safety....
Gooserider