When you download a HD file from the web it's going to be MKV format, this will not play on the ps3. It will play after you encode it or sometimes have to transcode it.
Alot of the HDTV mkv files i've came across can be encoded using MKV2VOb, free program, does a 8 gig file in about 2 minutes.
Just change the format to M2ts or Vob, say if you want the file to be spilt or not and your all done. I perfer M2ts myself compaired to Vob as you can rewind and fast forward the files, vob goes a bit funny.
If the file needs to be transcoded you will find out as soon as you hit add file as it will says Transcoding. Now This can take along time as it has to change the headers from DTS to Ac3, For Example a 4gig file could take upto 6 hours before it's complete.
but normally HDTV stuff from America from the BBC it will already be in the Ac3 4.1 format and will you just have to muxer the file with Mkv2vob before putting it onto dvd.
To burn the file just burn it as a date disk and use nero. What i have found tho is after the encoding mets sometimes won't fix on a single dvd so you have to cut the mkv, i.e take out the credits.
not had much sleep so hope that makes sense. if not just let me know where your having issues
i have been using mkv2vob for a few movies and all was going well but i have been trying to convert a new movie and keep getting an error converting from mkv to vob which has worked well in the past but now it gives me a message saying error extracting dts to wav and stops .