It should be easy but :
1. You have to have a cell phone capable of data services (most are)
2. You have to sign up for data services for your cell phone. This may be expensive.
3. You need to find a serial data cable for your phone, and almost all are now USB.
You would then configure your Tivo to treat the cell phone just like an external modem. Without flow control on the serial port, I wouldn't go faster than 9600. Most cell phones that support serial should support that baud rate. Phone numbers are:
*99***1# for GSM/GPRS (assuming it's configured as connection #1 in for mobile phone).
#777 for CDMA
There are, of course, many things that could go wrong with this setup. But I know in theory it should work.
Most people just give up and do PPP over serial or sell their Series 1 and get a Series 2. Turbonet is expensive.