This is a track which pushes engines hard. With its long straights Monza is a good test of outright speed to make comparisons between the years. Excuse my poor graph-producing abilities, but here's the evidence:
Have a look at the period from 2004 onwards. The big "blip" in 2008 was the very wet race (when Sebastian Vettel recorded his maiden F1 win) so should be discarded. However, the trend over the past six years is pretty clear and fits with the desire to cut the top speeds for safety and improved reliability.
These average speeds partly reflect the changes in engine regulations and their reliability. The 1990s saw big 3.5L V10 units in cars, before capacity was reduced to 3.0L in the mid-90s. These evolved until they were rumoured to top close to 1,000bhp by 2004.
By 2006, engines were further reduced to 2.4L V8 units which today pump out about 730-750bhp. Current engines are much more reliable thanks to advanced manufacturing techniques and an 18,000 rpm limit.
The next big change will come in 2013. Engines will be further reduced to four cylinder 1.5L units, but boosted by turbos again, mirroring those monsters of the 1970s and early 80s (where some reached 1,300bhp in qualifying format).
Worth noting too the return of Kinetic Energy Recovery Systems next year. By 2013 they'll be highly developed units and extremely efficient at retrieving energy from the braking cycle.