Not possible, at least until someone releases a homebrew app for cfw/jb ps3 that's able to do that, since in OFW sony never bothered to support srt/sub/txt separate subtitle files.
Best bet you have is to do as I do now and use AVIAddXSubs (google for it, it's easy to find) as it doesn't recode the movie but places the divx and the subs in a single .divx container file that the ps3 is able to read. Then open it, browse for the files (or folders) on your hard disk, assuming you already have the srt files with the same name for the movie files in the same folder, specify a folder or the root of your external HDD as the target and proceed.
That takes just a tiny bit more of time to place them there as it takes to simply copy the files with windows explorer, so no real hassle (on my P4 it's less than a minute for a 350 MB episode, less than 2 minutes for a 700 MB movie and so on). Just remember in XMB you have to press triangle to show all files on the external HDD.
Only problem is that this doesn't support MKV, only plain good ol' XVID/DIVX avi files. If you want MKV, best bet is to stream them like it was posted above as the codecs simply aren't compatible with the ps3 movie decoders. Like I said above, let's hope someone ports mplayer or vlc as a homebrew app sometime soon and you'll have what you want
PS3 Media server can embed subtitle files into the video (hard code them) on-the-fly as it is streamed from the PC to the PS3.
This is the easiest way to get what you want. Your only other real option is to manually convert the videos to get the subtitles burned into the video.
If your PC is fast enough to convert on-the-fly then PS3 Media Server is the best option and once setup its as simple as putting all your videos and their subtitles files in one shared folder, then selecting them for instant playback on the PS3.
You won't be able to just drop the videos on a drive, plug it into a PS3 and have it play all your movies and support for all the subtitles. The PS3 does not support enough formats or subtitle files natively to do what you want.
This is why PS3 Media Server was created in the first place. I just wanted to note real quick that PS3 Media Server does not change the original video files!
You will still have your original files on the PC, it just streams the converted version to the PS3, but still retains the original files on the PC.
Last edited by SwordOfWar; 03-14-2011 at 06:32 PMReason: Automerged Doublepost
Just a day after I answered here, a new homebrew app for the ps3 came out that is supposed to do what you want (although mkv support seems to be a bit buggy atm, but SRT subs seem to be supported): it's called showtime, so if you have a homebrew capable PS3 (custom firmware or jailbroken) maybe you should give it a try (I still haven't).