ISTP and INTP seem similar in some ways, and it's usually the interface of the Ti -- they both think soundly and structuredly. You can almost predict the logical path via which they'll approach a question. they both also have the same typical propensity for Fe neuroses and emotion disregard/stuffing.
So it's the secondary + tertiary where things differ.
ISTP = Se + Ni
INTP = Ne + Si
like others have said, ISTP (using Se) seems to collect more tangible data and focus on the data itself. INTPs are far more interested in the implications and possibilities of the data rather than the hard data. (In discussions, I've seen ISTPs far more inclined to get hung up on the specific numberical analysis of a real-world problem and not want to ballast that sort of info, while the INTP tends to conceptualize and generalize and work from very general principles drawn from the real-world data rather than with the data itself.)
The tertiary also has a big impact. Notable is the ISTP tendency towards distrust of social institutions due to the negativity of Ni ("Everyone is putting a spin on this situation; we're all tools of The Man; you can't trust anyone in authority.") It's almost like a kneejerk reaction sometimes. INTPs get more locked into Si ("This is the way the world is supposed to look, especially to fit with my theories, and based on what I know of the past.") Instead of bitching and grumbling about who might be trying to control them, INTPs just tend to withdraw and get absorbed in their ideas; but I think ISTP gets VERY hung up on the possibility that someone's messing with their autonomy or trying to fool them.
In any case, you can sense a more ethereal approach with the INTP (although not nearly as "flighty" as NF can come across, because theories are still being drawn from real-world observation and experience), versus a more carnal or tangible approach from ISTP.
EDIT: Also not sure how much this holds over, but I find that despite being SOOOO Ne and imaginative about the possibilities, I have this very very entrenched desire to have a secure base from which to operate. I hate having sensation and demands coming at me, I can't function. I just want to have a safe little haven, where I know what is what and I don't have to think about it.
I recently moved, and life has just gone to hell. Nothing is stable. It's driving me nutty, no matter WHAT my Ti+Ne perceives and comes up with; I just feel unsettled and unstable and nothing is secure.
This aligns with Si functionality (I see it in ISxJ a great deal!), but I don't see nearly as much from ISTP... or at least they don't seem to talk about it. They just seem crusty and secure no matter what they're thrown into.