Upgrading cassette - what do I need to be aware of? I'm looking to replace my rear cassette and reduce the tooth count between the sprockets in order to make my gear changes less dramatic.

I currently have a 11/26 9 speed SRAM cassette, and a stock shimano tiagra derailleur (whatever the 2011 trek 1.5 comes with). My chainring is a compact 50/34 110mm.

When I'm cruising along I often find that the jump between gears on the cassette is pretty substantial and it feels as though I could do with a missing gear to get the best efficiency with a cadence above 90.

I have my eye on a 12/23 cassette: http://www.wiggle.co.uk/shimano-hg50-9-speed-cassette/ (Shimano HG50). This is a fairly cheap cassette so it's not the end of the world if it doesn't work out, but what I'd like to know is if I can just replace the existing cassette with this one without having to buy a new chain/derailleur etc. Or, if I do need to buy some new stuff - what would that be? New jockey wheels?

I have all the tools to replace the cassette so that's not a problem. My concern is all the talk of 'hyperglide chains' etc.
As far as I'm aware, Shimano and SRAM cassettes are interchangable, at least in the 8 and 9 speed range. Your derailleur should be fine.  As far as the chain goes, depending on how worn it is, you might want to replace it with a new one. Mixing a worn chain with a new cassette is a sure way to wear out the cassette prematurely.  

Also, this is a good upgrade, as long as you don't do too many hills, as you'll be losing a few of the easier gears.  I recently did a similar switch, from 11-32 to 12-23 with an 8 speed groupset and noticed a huge difference. You aren't making as drastic of a change, so I'm not sure if you'll notice it as much as I did, but having a single step between adjacent gears makes finding the right gear quite a bit easier.