Per the FOMC announcement:
To help support the economic recovery in a context of price stability, the Committee will keep constant the Federal Reserve's holdings of securities at their current level by reinvesting principal payments from agency debt and agency mortgage-backed securities in longer-term Treasury securities. The Committee will continue to roll over the Federal Reserve's holdings of Treasury securities as they mature.
Here, we see an attempt by the Federal Reserve to prevent price deflation. Whether or not you believe price deflation will occur (I would wager that the readers of this website tend to view price inflation as the likely outcome), this particular move is inflationary as the Federal Reserve has not committed to reducing the size of its balance sheet.
Many intelligent people (e.g., Mish) are forecasting that price deflation will occur in the short-term. I tend to agree with Mish (in regards to the short-term). As noted in one of the Mises Daily articles for today:
After the crisis arrives and the depression begins, various secondary developments often occur. In particular, for reasons that will be discussed further below, the crisis is often marked not only by a halt to credit expansion, but by an actual deflation — a contraction in the supply of money. The deflation causes a further decline in prices. Any increase in the demand for money will speed up adjustment to the lower prices. Furthermore, when deflation takes place first on the loan market, i.e., as credit contraction by the banks — and this is almost always the case — this will have the beneficial effect of speeding up the depression-adjustment process.
The above quotation is courtesy of Murray Rothbard's book: Man, Economy, & State.