If you're not bothered about it being perennial then Cleomes could fit the bill. Also Dahlia "Bishop of Llandaff".
Then there's Leycesteria formosa which is a very fast growing hardy shrub.

What about some of the non-invasive Bamboos? You could always plant them in bottomless pots in the ground just incase.....

I would highly recommend Rudbeckia laciniata "Golden Glow"

It grows to about 7 -8 feet tall and flowers from July to late September. It is an evergreen herbaceous perennial. Just chop down the flowering shoots in autumn or spring and it will do its thing next year. It requires no dead heading... My kind of plant :^)

http://www.flickr.com/photos/31559373@N00/1224683654/

I've never thought of Cleomes, meanie - I'll give them a look, thanks.
Bamboo I have considered, but they are so expensive to purchase. I also thought of Ceonothus, but they are not totally hardy.
The Rudbeckia seems interesting. I didn't realise that there was one that grew that tall. I will investigate - thanks Anchorman.

Some bamboos grow quite quickly though don't they? So maybe you could buy younger plants. Ive never bought any myself so I'm not sure how expensive the they .....

With any screening plant you are going to have a cost. Bamboo can be purchased cheaply over the internet. A small plant will grow very quickly over this coming summer.

You could try Russian Vine. It grows very, very quickly and forms large, dense foliage. Please read up about that vine before planting though.

I wouldn't touch russian vine with a bargepole . It is the king of the thug plants. It is said that on warm day one can actually see it growing. It can reach 60 feet tall and if it gets into another tree it's a nightmare to remove.

Oh yes, the polygynum baulschuanicum or something similar - I once saw one that destroyed a farm outbuilding which was bigger than my house!!! Have no fear Anchorman. It's a lethal as Rosa filipes kiftsgate!!!
In terms of cost, I have got a Ceonothus that I grew from a cutting, so they don't get much more cheaper than that. Maybe I could give that a try.

Some of the evergreen cotoneasters make good screening plants. They grow about a foot a year some to a maximum of 6-8 feet.

One trim after flowering will keep them nicely in check

This link shows the one I have in my garden when it is in flower and there are a couple of further links from it showing it in berry and a bit about propagation.

http://www.flickr.com/photos/31559373@N00/3554081853/

Good idea that, Anchorman - if only I liked them. Don't know why, but Cotoneaster has always been a plant that has never inspired me. Perhaps I should consider it's functionality.

I'm only a month late to your quiery Jason, but here goes! Two years ago we planted a black bamboo and a ?green one. I think that they take a while to establish. The black one has grown, but it is not bushy; but the other is still quite small. I intend to put it in a large pot this year. They were quite costly and I wouldn't really recomend them. One of those yellow broom should be appearing in GS fairly soon, the one that we bought a few years ago grew amazingly in its 1st year. The Rudbeckia sounds interesting, I think I will try one myself. Grasses might be another good idea!

Thanks Driad - grasses are good, like my Miscanthus, but there is a small drawback. I'm presently trying to dig up a Miscanthus. My goodness, is it well rooted in. Forks and spades just seem to tease out the perimeter, and the big lump in the middle remains firmly entrenched. I shouldn't have watered it so much, I think. It went to 9 foot, but of course, throughout winter it was a spindly mess of brown shaggy sticks.
Bamboos sound good, eh, but finding one that doesn't spread, does 9 foot or so, and isn't expensive, is probably impossible.
Now, the yellow brooms - I grew two from seed last year, and they're doing fine in pots at the moment. Perhaps in a few weeks time, I'll plant them out. Thanks for the reminder.