It is compatible with the DSi - I've got one
HOWEVER - if you happen to have a DSi with a firmware version of 1.4 & you buy a Acekard 2i - you may find it won't work UNTIL you upgraded the acekard2i with the '1.4 fix' on the acekard website
You may be lucky that the company that you buy it off of may already upgrade the card for you (some will do it for a fee)
If not - you'll have to do it yourself but you will also need a DSLite or DSPhat to run the upgrade patch since you can't run the patch on a DSi with 1.4 firmware