Hi,
From the datasheet, section 5.3.5.1:
In several locations in the SFR bank, a single address is used to access two different hardware registers. In these cases, a “legacy” register of the standard PIC18 SFR set (such as OSCCON, T1CON, etc.) shares its address with an alternate register. These alternate registers are associated with enhanced configuration options for peripherals, or with new device features not included in the standard PIC18 SFR map. A complete list of shared register addresses and the registers associated with them is provided in Table 5-4.
My guess is that PBP handles this sharing/multiplexing of SFR adresses for you as long you use the _ALT designator when accessing the SFRs mentionend in the warning - but that's just a guess. If you're not actually using any of the SFRs mentioned in the warning there's obviously nothing to worry about.

/Henrik.