None of the memory expansion products were official Palm- they were all third party, and all had different ROMS on them, as the memory card (which resembled an SO-DIMM found in modern laptops) also had the ROM for the OS. (no wimpy EEPROMs there!) I never had one, but I do recall reading about them.
The springboard modules were initially the sole reason why I bought the handspring in the first place (well, along with the V3 of the OS), but never did end up using it. unfortunately, Handspring had some quality control issues as well, because my first one rolled over and died, and the second one eventually became completely unable to sync due to some issue with the dock connector internal to the device itself.
I also agree- PalmOS was... not well coded for multitasking. The hardware was nice and shiny, but the code probably could have been done better. And even the original palm had a fair amount of horsepower for what it was built for- Another app I recall playing around with drew a 3-D object on the screen and let you spin it using the touchscreen in real time, which was pretty impressive for that hardware.
Another vendor for USB-Serial adapters is Sealevel- We used them at work with one of our older cash counting machines and they worked quite nicely.