The Mitchell panel is a sham, of course, but the beautiful thing about a sham is that any development short of it being exposed as such (e.g. having an arbitrator say it's illegitimate) can be spun as a victory. After Giambi's day on the stand, as it were, let no man say that Bud Selig and George Mitchell didn't get to the bottom of whatever it is they wanted to be seen getting to the bottom of, and once this deal goes final, rest assured that MLB and the steroid witch-hunters in the media will crow long and loud about the "progress" being made.
It strikes me that Giambi is the big loser here. Before I say why, allow me to observe that life and ethics and stuff are complicated things. Personally, I don't buy in to the notion that one should keep mum about one's colleagues whatever the subject and whatever the cost, and indeed, there are times when one has no choice but to rat someone out. Until we're faced with the prospect, we can't really say what we'd do in such a situation. Should Giambi be talking to Mitchell? I tend to think he shouldn't, mostly because the Mitchell panel is an exercise in P.R., but I'm not Giambi so I won't moralize.
However, from what we've been able to divine about the Giambi-Mitchell negotiations thus far, it certainly seems like Giambi feels that ratting out others isn't a desirable thing. Indeed, if the reports are to be believed, the final piece of the deal is Mitchell's agreement to not ask Giambi about other players' steroids use. This, however, shouldn't give JG any comfort.
Yes, Giambi will have agreement saying that he doesn't have to give anyone else up, but it's not like Mitchell or MLB is bestowing some sort of transactional immunity to everything he ever said or did or will otherwise disregard the details of his testimony. Giambi is going to speak at length about all kinds of stuff, and the topics of conversation will naturally touch on trainers, friends, associates, and habits he shared with other players. Even if he doesn't have to answer a question such as "did you ever share a needle with Danny Tartabull," the information he provides will be useful ammo against others all the same, and everyone involved will know where the corroboration came from.
If that happens -- and it seems likely that it will -- will Giambi's testimony have technically thrown anyone under the bus? No. But it will have inadvertently bumped some folks off the curb, and he's going to have to live with it.