Update (some thirteens years later): It is now as easy as it seems if you use for example the wonderful and powerful r2u system I set up last year, and which now provides over 20k binary .deb packages for each of twi Ubuntu LTS releases (currently: 20.04 and 22.04), and is also accessible via install.packages() thanks top bspm. Follow the link to r2u for more.
It's not as easy as it seems.
On my Ubuntu machine at work, I go with the second solution. But because the first one is better if you have enough coverage, we have built cran2deb which provides 2050+ binary deb packages for amd64 and i386 --- but only for Debian testing. That is what I use at home.
As for last question of whether you 'should you expect trouble': No, because R_LIBS_SITE is set in /etc/R/Renvironment to be
# edd Apr 2003 Allow local install in /usr/local, also add a directory for
# Debian packaged CRAN packages, and finally the default dir
# edd Jul 2007 Now use R_LIBS_SITE, not R_LIBS
R_LIBS_SITE=${R_LIBS_SITE-'/usr/local/lib/R/site-library:\
/usr/lib/R/site-library:/usr/lib/R/library'}
which means that your packages go into /usr/local/lib/R/site-library whereas those managed by apt go into /usr/lib/R/site-library and (in the case of base packages) /usr/lib/R/library.
Hope that clarifies matters. The r-sig-debian mailing list is a more informed place for questions like this.