Reading through the TeX Live manual, the best way to achieve what you want seems to be to alter where TEXMFHOME points to. This is where TeX looks for your personal files, and has standard setting ~/Library/texmf, as you probably know!
At the Terminal, typing
sudo tlmgr conf texmf TEXMFHOME "~/texmf"
will alter the location to ~/texmf, which would be the normal setting on Unix systems other than a Mac. Note that this alters the setting system-wide, but I'm guessing that for most people this will be acceptable.
If you want to have more than one possible location (say retaining the standard setting and adding another one), then a colon is used to separate the list:
sudo tlmgr conf texmf TEXMFHOME "~/Library/texmf:~/texmf"
By the way, for recent releases of TeX Live you don't need to run texhash for your local tree. The database is only required for the 'big' installation tree, while the local tree will be scanned when TeX runs. The assumption is that the local tree will always be relatively small, so this is an acceptable performance hit.
There are (at least) two parameters you may want to set, and two ways to set them (env var vs conf file) and finally you might want the setting to be system-wide or user specific. Since all this is more or less orthogonal, it makes like 8 possibilities.
First, you may change either TEXINPUTS or TEXMF. The former acts only of TeX's input search path, the later is also used by other tools (imagine the tree contains fonts, bibtex or makeindex styles, etc.) With TEXINPUTS, you should append (or preppend, if files from the new tree should override other versions) /usr/share/R/share/texmf/tex//, separated with a :. For TEXMF, should need to insert the same path where you want inside the braces in the current definition.
Second, environment vs configuration file. For the environment, adding something to TEXINPUTS is easy, but for TEXMF you need to copy the full definition, so that it may look like TEXMF='{$TEXMFCONFIG,$TEXMFVAR,$TEXMFHOME,$TEXMFR,...,$TEXMFDIST}' (with an appropriate definition of a TEXMFR environment variable). Editing the configuration file goes the same for TEXMF, but it's less convenient if you want to change TEXINPUTS since you also need to edit the various TEXINPUTS.format for each format you intend to use. With and envvar, it is easier, since setting TEXINPUTS=:$TEXMFR/tex// in the environment will insert the appropriate TEXINPUTS.format from the conffile before the leading colon.
Finally, user vs system. If you chose to use envvars, I refer you to your favourite shell's (and desktop environment) documentation. For configuration files, it should be noted that, system-wide, there are usually two texmf.cnf files which are both read. You should edit the first one returned by kpsewhich texmf.cnf (in case of doubt, look at the comments in the file). Include only your definitions, eg:
TEXMFR = /usr/share/R/share/texmf
TEXMF = {$TEXMFCONFIG,$TEXMFVAR,$TEXMFHOME,!!$TEXMFSYSCONFIG,!!$TEXMFSYSVAR,!!$TEXMFMAIN,!!$TEXMFLOCAL,$TEXMFR!!$TEXMFDIST}
(this is my preferred option btw). Note that if you use a packaged (by Debian or other) version of TeX Live, this may change, see Sylvain's answer. If you want to proceed for a single user, it's harder, because you should place a full (edited) copy of the original texmf.cnf file in some place (~/.texlive2010/texmf-config/web2c would be a ratonal choice) and defined an environment variable TEXMFCNF pointing to that directory.
I hope I didn't got you confused with too much choices, but I thought it might be interesting to explore the different options. If you have admin rights, I'd recommend editing the system-wide additional texmf.cnf to define a TEXMFR and adding it to the definition of TEXMF, as explained above.
Best Answer
The LaTeX style files for TeXLive on Windows are found in
where WXYZ is the year.