I've (apparently) got my computer all set up with my SPDIF plug made, got it plugged into my TV and mplayer is working great. It looks and sounds so good, that I forget I'm not watching my DVD player and I reach for a remote. Right now I have to get up and go to the keyboard.

Now, a wireless keyboard might be the solution here, but wireless keyboards are big, I mean what are you going to do with it while you watch the movie? Whereas I like to keep the remote in my hand with my thumb on pause and stuff. I have an 'IR blaster' device which consists of an IR detector terminating in a 3.5mm audio plug; I understand it's for TV cards, though. Is there any way I can use it with a generic remote for controlling mplayer?

Can't quite tell what you have, but if it's an IR receiver you can probably make it work if your MB has a serial port and you don't mind rewiring it. Might be better just to buy a receiver that's known to work though. You can pick one up for about $5. And you can use any IR remote with it as long as you find or create an lircrc file for it.

I don't have a serial port; I guess that means I have to buy a USB IR reciever. That's good news though, I'll have to get that set up.

Just pick a receiver/remote supported by lirc-
http://www.lirc.org/

See left hand side, scroll down. The list might be out of date, and I think lots more remotes are supported.

Lirc is the "standard IR API" for Linux.

Plenty of USB receiver/remote combos supported- the Streamzap and Windows MC combos are supported and commonly available for $30 or less both online and in stores.

Well, one thing, my computer isn't up front by my TV. So if I got a USB IR reciever, I would have to have a long USB cable to get it up by my TV.

Second thing, my stereo reciever, which I don't have a remote for, has a 'remote control' jack on the back. Does this mean if I point any IR remote at my stereo reciever, with it on, it basically functions as an IR reciever (I assume if the remote issued commands the stereo reciever understood, it would execute them, but if not, it would pass them through anyway). This still doesn't do me any good, though, because I don't have a serial port; I have heard of sound card implementations, but I'm not sure about that.

Well, one thing, my computer isn't up front by my TV. So if I got a USB IR reciever, I would have to have a long USB cable to get it up by my TV.

Second thing, my stereo reciever, which I don't have a remote for, has a 'remote control' jack on the back. Does this mean if I point any IR remote at my stereo reciever, with it on, it basically functions as an IR reciever (I assume if the remote issued commands the stereo reciever understood, it would execute them, but if not, it would pass them through anyway). This still doesn't do me any good, though, because I don't have a serial port; I have heard of sound card implementations, but I'm not sure about that.

You can add up to 15ft of USB extension without issue, more with an active extension cable or powered hub.

Another point- even though your motherboard may not have a serial port on the back with the other ports (USB, PS/2, Ethernet, sound, etc), it may have a header on the motherboard for a serial port ribbon cable/DB9 assembly/bracket. Check your BIOs or manual for mention of a COM serial port connector or enable/disable.