I know the Ultegra is listed as having a 27t max sprocket but will it take an 11-28 Scram 10 speed cassette? Anyone tried it?

it works. there is a little clatter from the top pulley riding on the top cog a bit. if you're not incredibly picky about noise, you'll be fine.

that said, that sram 11-28 is not as smooth as a shimano 12-27. it's a small compromise if you need the gear, though.

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Shimano have included an 11-28 10 speed Dura Ace cassette stack to their inventory. Works beautifully with sram compact such as the Red group set which has had huge global shifting problems. I'm not sure if it's available in Australia yet, if not it won't be far off as Shimano Aust are doing a 2010 New Product Launch early next month. It's only a matter of time before it's also available in Ultegra. Maybe part of the launch.
A complete new Ultegra group set is being offered by Shimano for 2010 more akin to dura Ace in appearance, in particular STI looks and more. Don't know much more till the launch in August.
With a conventional 39/53 set up Shimano recommend 27 as the lowest ratio due to jockey wheel contact with short cage rear derailleur hence you will get more noise although shifting should and will still be fine.
I can tell you with great certainty that Sram cassette stacks have been problematic. I would suggest you avoid this as an option until Sram own up to the shifting problems that they have been experiencing and fix the internal spacing issues within the cassette stacks. I know that Shimano Aust know of these problems and are having a quiet smirk at Srams expense. A top end pro store in the Pyrenees in France have told me that they've had (by his words) an 80% rate of problems with said item. They are trying not to sell Sram in consequence.
Shimano's Dura Ace is maybe over priced but it works unconditionally. Yes you've heard it before but you do get what you pay for. Someone's got to pay for the R&D and it is made in Japan using cold forging/welding as apposed to Taiwan, although Taiwan on the whole make better than exceptional quality components and frames etc...
Big news is that next Shimano container to arrive in Sydney will see prices decrease noticeably due to 80 cent to the US $.
Hope this helps.
Best of luck
Ps If Shimano stacks works with Sram then Sram should work with Shimano. It's all about the spacing between cogs.

Yep, works fine.

Is this only with 10 sp SRAM cassettes? The reason I ask is that I've been using SRAM 9 speed cassettes with my Ultegra shifters and DuraAce RD for years and they shift flawlessly. I did have an Ultegra 9 spd cassette a couple of years ago, but didn't notice any difference between it and the SRAMs.

The SRAM-Shimano spacing between cogs on 7/8/9 is identical. SRAM chose their own spacing on the 10 speeds instead of adapting Shimano so in indexed mode only its a problem on 10 speed.

I've had it for 1 year. Ultegra Compact (50/34) drivetrain with SRAM 11/28 10 speed cassette. Great combo, smooth as silk. Great for steep hills and rolling terrain. Also plenty of gear for flats (at least for me). It does take a little more careful tuning of indexing to get it just right...

It's both a 10 & 9 speed issue.Obviously 10 speed for road and 9 speed for MTB.We have had to replace a good twenty odd 9 speed cassette stacks from new MTB bikes in our store alone due to poor shifting at the assemblage stage. 10 speed road has also been a problem however I can't comment on Sram 9 speed road as we didn't see too much of it nor did we hear any problems about it in the shadowy resesses of the bicycle industry....

Have not had that problem...(10 speed) I did have a little noise at the very beginning but I tweaked the RD and after that it's been smooth and quiet.