My understanding is that Apple's goal is to use the SSD for as much data as it can handle. The way Ars describes, you're using the HDD to augment the capacity of the SSD, as opposed to standard caching, which uses the SSD to augment the speed of the HDD.
Jury is still out on some of the technical details, but generally speaking, writes always go to the SSD first. It will keep writing to the SSD until it is "full". I know that at least 4GB is reserved for a write cache on a 128GB drive. This may be a fluid amount based on the size of the SSD, I'm not sure. I also don't know if any other space is held back for any reason.
Once the write cache is filled, if you're still writing data, it will seamlessly spill over onto the HDD. Over time, the OS will determine what data would benefit from being "promoted" to the SSD, and what data can be demoted, and uses idle time to rearrange the data.
This should speak quite a bit to the kind of SSD (and controller) you should get. If the OCZ drives suffer from degraded performance when they are only 50% full, I would highly suggest not using that kind of drive in a Fusion setup. (I also wouldn't recommend that kind of drive at all, because that's a ludicrous limitation to have on any storage device.) Again, philosophy matters here. Your primary storage is always the SSD, and all writes will go to the SSD, so write speed is going to be a very important stat. Likewise, most data will be read from the SSD first.
That said, I'm using a relatively low performing drive (Crucial M4) compared to some on the market, but it was cheap, and still a massive performance boost compared to the stock 5400RPM drive, so even a mid-quality drive should yield positive results.
As to your other questions, TRIM and garbage collection are two sides of the same coin. In most cases, the firmware of the SSD will take care of those issues. In my case, the Crucial M4 really needs to have TRIM enabled, so I ran the TRIM Enabler at Groths, and that did the trick. Garbage collection is handled by OS X, as far as I'm aware. Generally speaking, though, the lifespan on these drives will likely outstrip the useful life of the computer itself, so I wouldn't stress this too much. Find out if your drive needs the TRIM Enabler (a Google search should do it), and after that, you'll just need to trust things are working as they're supposed to.
Thanks for this.