Copdriper, you ask, "Ok...but won't the PSX shareholders receive some sort of compensation for part of their original investment in PSX (the transportation assests) being sold off in an MLP IPO...?"
If the MLP IPO offering and subsequent business are successful, then PSX shareholders should benefit, but not from the issuance of MLP shares, cash, or other tangible assets.
When the IPO occurs, PSX will sell shares of the MLP to the public. The money raised from this sale will go into PSX's assets, giving PSX more assets for capital investment, making it a more valuable company, and, hopefully, raising it's pps. Furthermore, I suspect that PSX will hold on to some shares of the MLP, perhaps a majority of the shares. If this is the case, then PSX and it's shareholders will benefit, pps-wise, if the MLP is successful and brings in nice profits.
In summary, PSX shareholders can hope to profit from the MLP by money being brought in to PSX from the IPO and from profits that the MLP generates in it's business activities.