You just need to have the rebate postmarked within 30 days of purchasing the memory. As long as your invoice is dated within the month of January, you are all set.
I have done 3 or 4 Kingston-based rebates so far, and have received every one of them without a problem. Just be smart and keep a copy of the filled-in rebate form, packing slip, and UPC.
I ordered one for me to take myself up to 384MB. I also ordered one for the mother-in-law. She is still running at 32MB on Windows 98 - good thing she called me last night and asked why her computer was running so slow (that and she only had 6MB of hard disk free).