The Linux kernel supports TRIM as of version 2.6.33.
So the answer is: Yes, Linux, in it's latest version, supports TRIM out of the box. Ubuntu 10.04 doesn't though.
The question is whether the feature will be back ported to 2.6.32 or 10.04 will get an update to 2.6.33+. (see João Pinto's answer)
Of course your SSD has to support it as well. There are also still some issues with some specific SSDs (I think the Indilinx ones may still be a bit wonky for example). The SSDs firmware has to implement everything properly - but, I understand, some SSDs implement TRIM in a way that isn't really conform with the ATA standard. Or at least in a buggy way.
The first generation X-25M from Intel doesn't support trim "Properly" (or not at all - strictly speaking). The second generation Intel X-25M G2 does. I don't know whether the latest firmware adds proper TRIM, the Intel website keeps mentioning it's "Optimised for Windows 7", which could mean it's bolted on as a software feature in the windows driver, and that the disk doesn't do it on it's own because Intel left their homework on the bus again :-)