OSX readyboost?

No. USB flash drives can only be used for data storage. OS X has superior memory management to that used in Windows and utilizes disk-based virtual memory on your hard drive. In most cases if your computer actually needs VM this would be faster than a USB flash drive which is considerably slower than your hard drive.

ReadyBoost isn't used as a VM replacement. Readyboost is more like a disk cache for small file access. Since most USB key have a seek time of 1ms it is faster to cache small file on the key and access them from there than looking for them on your HD that has a seek time of 8 to 12ms. The caching of big file wouldn't be cost efficient since the transfer time is slower from the usb key than on your HD but for really small file you have a gain with readyboost.

Benchmarks I've seen of ReadyBoost do show that it improves performance somewhat, but in many if not most circumstances not near as much as just adding RAM to the system. When compared to the high cost of flash RAM, adding internal memory is usually much more cost-effective and will overall result in higher performance gains. So the lack of a ReadyBoost-like feature in Mac OS X probably isn't a big lack at this time. The only apparently significant advantage of ReadyBoost-type technology is the ability to connect a flash drive to a system you don't own where you cannot add internal memory.