I can see a memory unit upgrade on the Arcade. No way will they put a 20 GB HDD on it though -- short themselves some serious cash on the $100 60 GB pack, while increasing their losses on each Arcade SKU? I don't think so. That'd be pretty knuckleheaded of them.
I also don't think they're going to bundle a HDMI cable with the Pro -- again, cutting their per/unit profit... not a good idea. HDMI cables are not that cheap to manufacture -- they'd be better off upping the HDD to 80GB (something they can talk about in big digits on the box), or even bundling software -- and they certainly aren't going to do all of that.
MS's profits are from software, not hardware -- they aren't going to make the hardware thorn bigger, they'll just scale it up to match the PS3 bullet points (80 GB drive), and still keep the price low, without shooting themselves in the foot.