In a precisely worded forum post at Halo.bungie.org, Bungie community rep Matt Soell has referred to the claims of the demise of PC/Mac Halo-- centered mostly on reports of the cessation of online preorders at Electronic Boutique's web site-- a load of bollocks .
In other words, as usual, the information that retailers give is often for their own reasons, and is scarcely to be trusted. Heck, if memory serves, some other vendors still were claiming Halo would be released about this time, so everyone can just take a Prozac and relax; Halo is still coming.