Interesting... Personally, I think it's very significant. Every spec I've seen shows PC2-5300, and so frankly that constitutes false advertising. For example, from CDW's listing:
Configuration Features2 x 1 GB
Installed Size2 GB
Max Supported Size2 GB
Memory Specification CompliancePC2-5300
Memory Speed667 MHz
TechnologyDDR II SDRAM
If I get mine from CDW and it's PC2-4200, then I'm going to have an issue. Note that I'm not prone to attack businesses--in fact, I'm very pro-business. But, this really surprises me...
I've noticed that some vendors (e.g., Gateway) indicate that selecting PC2-5300 RAM will delay shipment (as opposed to PC2-4200), so perhaps there are shortages. However, it would have to be CLEARLY indicated that the RAM doesn't meet spec, or else it becomes, again, false advertising. And, it's not insignificant--support for faster RAM is one of the key selling benefits of the 945 chipset, and it does make a _significant_ difference in performance.
If I were anyone with this RAM, I'd be making a serious stink. At least, I'd be demanding that Acer provide PC2-5300 RAM at the earliest convenience, and pay for shipping in both directions (including shipping the PC2-4200 RAM back). You simply haven't received that you paid for...