EDIT1: Below I have compiled the data for Ulduar Hard Modes.
First I'm just going to make direct comparisons without any analysis.
Sunwell Plateau
Kalecgos
Released: 25th March 2008
Downed: 25th March 2008
Days: First Day
Brutallus
Released: 25th March 2008
Downed: 25th March 2008
Days: First Day
Felmyst
Released: 25th March 2008
Downed: 26th March 2008
Days: 1 Day
This is where the gating system was implemented...
Eredar Twins
Released: 8th April 2008 (9th April in EU)
Downed: 10th April 2008 (EU Guild)
Days: 1 Day
M'uru (a.k.a the "hardest" boss eva)
Released: 29th April 2008 (30th April for EU)
Downed: 4th May 2008 (EU Guild)
Days: 5 Days
Kil'jaeden
Released: 20th May 2008 (21st May for EU)
Downed: 25th May 2008 (EU Guild)
Days: 4 Days
Sunwell Plateau is often described as the "Hardest" Raid in WoW. It was the final raid of "The Burning Crusade", an expansion that is regularly compared to WotLK as a means to say "Wrath is too easy and TBC was much harder". Here is the data for ICC.
Icecrown Citadel: Hardmodes
Lord Marrowgar
Lady Deathwhisper
Gunship Battle
Deathbringer Saurfang
Valithria Dreamwalker
Festergut
Rotface
Blood Princes
Blood Queen Lana'thel
Released: 10th February 2010
Downed: 10th February 2010
Days: First Day
Sindragosa
Released: 10th February 2010
Downed: 11th February 2010
Days: 1 Day
Professor Putricide
Released: 10th February 2010
Downed: 12th February 2010
Days: 2 Days
The Lich King
Released: 10th February 2010
Downed: TBC
The numbers are quite similar to SWP with the obvious difference being M'uru and Kil'jaeden taking slightly longer. Apart from those two bosses, the rest of the bosses in SWP took roughly the same time to down as the Hardmodes of ICC. Time will tell when The Lich King will be downed, but he needs to survive for just 3 more days to last longer than M'uru and Kil'jaeden.
An interesting note, The Lich King normal encounter took 2 days to down, which means he took longer to down than the first 4 bosses of SWP (and most of the ICC hardmodes for that matter). Now, that may be contributed by the fact that there were limited attempts for the fight, but the cock-block limitation was largely softended by those guilds using multiple alts. That being said, it's probably still a bit of an unfair comparison.
Conclusion: So is ICC "Faceroll" easy because a few top guilds have downed so many Hardmodes already? If that's the case, then we can assume SWP was just as easy, since most of it's bosses took no longer than a single day to down.
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EDIT
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As suggested, here's the data for Ulduar Hard Modes. Since there were no limited attempts in Ulduar, we can easily make a direct comparison to SWP.
Ulduar: Hard Modes
All Hard-Modes were technically available as soon as Patch 3.1.0 hit (14th April 2009) but in order of fairness, it's probably best to say the Hard Modes were only truly attempted after the first reset (21st April 2009), since most guilds were focused on clearing the Normal modes.
Flame Leviathan
Available: 14th April 2009
Downed: 14th April 2009
Days: First Day
XT-002 Deconstructor
Attempted: 21st April 2009
Downed: 20th May 2009
Days: 29 Days
Note: XT-002 received a lot of nerfs and really wasn't tuned properly at the start of 3.1.0 (though the same can be said of M'uru)
Assembly of Iron
Available: 14th April 2009
Downed: 20th April 2009
Days: 6 Days
Hodir
Available: 14th April 2009
Downed: 19th April 2009
Days: 5 Days
Thorim
Available: 14th April 2009
Downed: 16th April 2009
Days: 2 Days
Freya
Available: 14th April 2009
Downed: 17th April 2009
Days: 3 Days
Mimiron
Attempted: 21st April 2009
Downed: 1st May 2009
Days: 10 Days
General Vezax
Available: 14th April 2009
Downed: 17th April 2009
Days: 3 Days
Algalon
Available: 1st May (when Firefighter was completed)
Downed: 3rd May 2009
Days: 2 Days
Note: Could only be attempted 1 hour per week, so it's safe to say Algalon was defeated in a couple of attempts and not truly in 2 Day
Yogg +0
Attempted: 21st April
Downed: 7th July 2009
Days: 77 Days
From the Data above, around 4 boss encounters in Ulduar took longer to down than it did with M'uru. It's quite clear that Ulduar Hard Modes took much, much longer to complete than Sunwell Plateau. Since there were no limited attempts in Ulduar, its a much more fair comparison. To anyone who says Wrath is easy and things were much harder with TBC because of SWP, you may want to redirect them to the data shown above.