We can see that real rents have been appreciating between 0.75% and 1.25% per year. Note that from between 2006 and 2010 population growth was on the high end of historical ranges and this is correlated with tightness in the rental market.
Looking at the rent to median income ratio:
Median incomes have fluctuated more than rents, causing the ratios to be more variable. After 2008 it can be immediately seen that as a percentage of income rents are now more dear.
An interesting graph is tracking dwelling spreads -- the percentage change between a dwelling type and the dwelling type with one additional bedroom:
What this graph shows is a "curve" for the spread between different dwelling types, in some ways akin to a bond yield curve. A diminishing spread would indicate compression between dwelling types.
There had been some thought that real rents have not been increasing. These data indicate that real rents are increasing at a rate around 1% per year, with more marked changes happening recently from 2006 through 2009. In terms of incomes there is additional fluctuation due to labour market changes, however it appears that rents are over longer time periods tracking incomes closely, though 2011 has produced tighter conditions.
Changes in population growth appear to be having an effect on rental rates. It is unclear recent tightness in the rental market is a permanent change -- for example, rents are increasing because the city is comparably more desirable for a given income -- or if we should expect relative weakness in rental growth going forward. The most recent rental survey from April 2012 indicates no strong signs of weakness.