I am now aware that the 4GB sdhc isn't compatible. Are there any sdhc cards compatible with the Mark IIN. Or am I stuck with plain 2 GB SD card?

actually- i'm pretty sure that there was a firmware update that addressed this issue?

Update to the latest firmware, SDHC will be supported. The write speed is slow, but at least it works.

Yep, you need to update your firmware to 1.1.2

http://web.canon.jp/imaging/eos1dm2n/eos1dmk2n_firmware-e.html

Agree to all of the above and use 8 gb SDHC cards with no issues.

While the firmware update will make it work, the speed with SDHC is significantly slower than regular SD (or CF). This is worth keeping in mind.

They're right about the new firmware - I've been using SDHC for about a year with no problems.

As for the speed - I have not seen timings for the 1dm2n specifically, but the data rate is similar between SD and SDHC cards in the 1dm3 - both of which are faster than the CF cards. Granted, the 1dm3 is quite different and faster than the 1dm2n, but there is nothing inherently different between SD and SDHC cards...
http://www.sdcard.org/developers/tech/sdhc/

http://robgalbraith.com/bins/camera_multi_page.asp?cid=6007-9038
From Rob's Database (timings for 1dm3)

Overall, SD is fastest, followed by SDHC and CF is slowest
Fastest SD - 10.338 MB/sec JPEG and 14.795MB/sec RAW
Fastest SDHC - 9.972 MB/sec JPEG and 13.692 MB/sec RAW
Fastest CF - 8.499 MB/sec JPEG and 10.901 MB/sec RAW

Compare that to the fastest card for the 1dm2n (database has not been updated with new firmware - no SDHC)
http://robgalbraith.com/bins/multi_page.asp?cid=6007-8200
Again, SD cards were faster than CF -
SD 7.370 Jpeg Quality 8 and 8.668 for RAW
CF 6.468 Jpeg Quality 8 and 7.674 for RAW

The 1d3 is significantly faster than the 1dm2n.
I find it difficult to believe that the SDHC card in the 1dm2n is significantly slower than the SD or the CF card.

I'd love to see some updated timings, though.

Of course, the normal caveat - YMMV.

Recent SDHC cards are a lot faster than the early ones. When the firmware for the 1D2 and 1Ds2 was upgraded the SDHC cards were noticeably slower than the SD cards of the day and also the CF cards of the day. I haven't bothered with SDHC as I now concentrate on CF cards. I made that decision when the SDHC cards were too slow and the SD cards were too small. In future I will use UDMA cards that are supported by newer cameras.

Something in the memory card management of the 1D2 series makes it incapable of getting the best out of any modern cards.

- Alan

Actually Alan, it is not just the 1d2 series that do not get the most out of modern cards. The idsm3 is pumps data faster than its high-speed sports counterpart - at 16+ anf 19+ on a CF card - and 13+ and 16+ on SD card.

Compare that with the 50d - 24.6 jpeg, and 33.7 for RAW (CF card). The Digic 4 does have some speed. And compare that with the 40d - 8.7 jpeg and 11 RAW.

Compare the Nikon d90 - the data rate is 18+ for jpeg, and 20+ for NEF RAW. That is one of their entry level cameras. The d700 is pumping data at 23+ for jpeg and nearly 30 for NEF RAW. The d3 clocks in about the same.

The Sony a900 is pumping data at 31+ jpeg and 35+ raw.

To say the Canon gear (before the 50d) is "...incapable of getting the best out of any modern cards" is charitable.