Bill Gates hinted to Japanese reporters Thursday that Microsoft might be interested in licensing the Xbox software to third parties. The move could turn the Xbox into a platform, with clones flooding the market and giving Microsoft more leverage to oust Sony's PlayStation as the dominant gaming console. According to a Microsoft spokesperson, "the basic software" for the Xbox system would be offered. However, no formal decision has been made as of yet. The licensing agreement could be similar to the way Microsoft licenses Windows to computer makers, but Gates offered no specifics as to who may use the Xbox software. Source: BetaNews
Personally I think it won't matter too much if you can install third party software on the Xbox or not. Since the PSX fans will buy the ps3 and Xbox fans the Xbox 2. But considering, currently you are unable to go online if you mod your Xbox! Allowing third part software would be a major advantage, since you to add more features than the ps3. So it might have some affect but not likely to change some ones mind. I'd probably get both since each console has exclusive rights to certain games.