Is there a way to burn into a virtual CD-R instead of burning into a real one? There is some .m4b files from the iTunes store that I bought, and from what I understand, these files cannot be converted into .mp3, unlike the .m4a files.

To put those files into another audio player that I have that can only support mp3, I need to convert it... and it looks a way to do it is to burn the .m4b into a CD-R, by first creating a playlist, and then burn the playlist into a CD-R, and there are up to 7 times that can be done.  Then, once the CD-R is made, then it can be ripped as an .mp3

But since I don't really need a CD-R version, so an extra CD-R will be wasted.  Is there a way to burn the .m4b onto some virtual CD-R and then rip it instead?  (for Snow Leopard or Lion).  But if it involves installing questionable shareware that might slow down the machine overall, it may be not as desirable -- so if it is shareware, it'd better be a reliable and trustworthy shareware.

(as a side note, the .m4b protection doesn't seem very strong... since anybody can just burn it and rip it, and then it is DRM free... the cost is only about 10 cents for CD-R, for about 16 songs... so the use of .m4b as a protection together with the ability to burn it onto a CD-R is somewhat strange to me)

Update: it seems like we can use a CD-RW and write it and erase it... and there won't be any wasted CD-R...  the only thing is I don't know how many times they can be erased...
While you can write data to a .iso file in Disk Utility, I don't believe there's any way to do so with the burning functionality built in to iTunes.

You could however get a program like Virtual CD-RW, which presents itself as a CD-RW drive to the OS, which iTunes could then burn to. The reviews for that program don't look great, so I'd suggest getting the demo version and seeing if it works for you before paying any money.

Roxio's Toast Titanium may include similar functionality, but I couldn't find any definite confirmation of that on their site.